…nd dist

The CLI launcher derives `_PACKAGE_ROOT` from where `unsloth_cli` imports
from, and `studio/backend/run.py` derives its default `frontend_path` from
`Path(__file__).resolve().parent.parent / "frontend" / "dist"`. When
another `unsloth` (a separate venv with `pip install unsloth`, a system
install, an older venv earlier on PATH) wins `which unsloth`, both
resolve into a site-packages tree that ships frontend source files but no
vite-built `dist/`. The backend warned `[WARNING] Frontend not found at
...` and then happily served 200 on every `/api/*` route while returning
`{"detail":"Not Found"}` on `/`. The 404 was silent to users -- the
process was healthy, the log line scrolled by, and the only symptom was a
blank browser tab.

This is a real situation: many devboxes carry a workspace venv with
`unsloth` installed years before the user runs `curl|sh` to install
Studio. The installer-managed binary at `~/.local/bin/unsloth` exists
but loses to the older venv on PATH order.

Three layers of fix, additive:

Layer C -- runtime auto-discovery (unsloth_cli + run.py)
The CLI now resolves `--frontend` explicitly before spawning `run.py`,
probing in order: package-local default, installer venv site-packages
(`$STUDIO_HOME/unsloth_studio/lib/python*/site-packages/...` and the
Windows `Lib/site-packages/...` equivalent), and editable-install source
roots read from `__editable___*_finder.py` MAPPING dicts in the installer
venv. `run.py` does the same probe as a backstop for direct `python
run.py` invocations.

Layer E -- loud structured error
The silent `[WARNING]` is replaced with a `SystemExit` that names every
candidate path tried and lists the four one-line fixes (run the absolute
path, pass `--frontend`, pass `--api-only`, reinstall). Suppressed only
in `--api-only` mode where no UI is served by design.

Layer F -- installer self-check (install.sh + install.ps1)
At the tail of install, both installers compare `command -v unsloth`
(POSIX) / `Get-Command unsloth` (PowerShell) against the just-installed
binary. If a different path wins, a yellow `warning` block names the
shadowing binary and prints the alias / absolute-path / PATH-reorder
fixes. install.sh uses the venv Python for path canonicalization so it
also works on macOS (BSD `readlink` has no `-f`).

Cross-platform notes:
- Glob patterns probe both `lib/python*/site-packages` (POSIX) and
  `Lib/site-packages` (Windows).
- Canonical-binary path branches on `sys.platform == "win32"` to pick
  `unsloth.exe` over `unsloth`.
- install.sh fixed for macOS; install.ps1 is the Windows analog.

Tests: `studio/backend/tests/test_frontend_resolution.py` covers five
cases via AST-load of the helpers (no uvicorn / FastAPI import needed,
matching `test_host_defaults.py`'s style):
1. Resolver returns None when nothing exists anywhere.
2. Resolver picks the first existing candidate when the default works.
3. Fallback to `$UNSLOTH_STUDIO_HOME` site-packages dist when the default
   is missing.
4. Fallback to an editable-install source root via MAPPING parsing.
5. Resolver tolerates a non-existent `$UNSLOTH_STUDIO_HOME`.

All 5 new + 2 existing host-default tests pass.

…h hint, broader tests)

Four parallel platform reviews (Windows, Linux, macOS, general) on the
initial commit surfaced a small batch of correctness items, all addressed
here:

Windows install.ps1 (medium severity, false positive on every install):
The user-facing shim at $StudioHome\bin\unsloth.exe is a hardlink to
$VenvDir\Scripts\unsloth.exe (created at line 1582). Resolve-Path does not
de-duplicate hardlinks, so the previous string compare always saw the two
paths as different and the new "another 'unsloth' wins on PATH" warning
would fire on every fresh Windows install. Switched to content-hash
equality via Get-FileHash, which collapses hardlinks, symlinks, and
identical copies to a single identity. Also restricted the probe to
Get-Command -CommandType Application so PowerShell aliases / functions /
scripts named "unsloth" don't false-trigger.

Windows run.py SystemExit hint (medium severity, defeats the recovery UX):
The structured error printed Path(STUDIO_HOME)/"unsloth_studio"/"bin"/
"unsloth.exe" on every platform, but on Windows the installer places the
shim at $STUDIO_HOME/bin/unsloth.exe (no unsloth_studio segment) and the
venv binary at $STUDIO_HOME/unsloth_studio/Scripts/unsloth.exe. The hint
pointed at a non-existent path on Windows. Branch on sys.platform ==
"win32" to emit the real shim location; Linux / macOS keep the unsloth_
studio/bin/unsloth layout.

MAPPING regex robustness (low):
[^\n]* silently failed if a future setuptools / black reformat wrapped
the MAPPING dict across multiple lines. Tightened to [^}]* + re.DOTALL,
which still rejects nested dicts (setuptools never emits those for
editable installs) but tolerates either single- or multi-line literals.

install.sh broken-venv edge case (low, macOS reviewer):
Previously _canon fell back to echoing the raw input when the venv python
failed, which would make two symlinked-but-identical paths look different
and false-trigger the warning. Now _canon returns empty on failure and
the caller skips the whole comparison if either side is unresolvable.

argparse default + log readability (nits):
run.py's argparse --frontend default now reuses the module-level
_DEFAULT_FRONTEND_PATH constant so it stays in lockstep with run_server's
default. The [OK] log message resolves the chosen path so support output
is always absolute.

Tests grow from 5 to 8 in studio/backend/tests/test_frontend_resolution.
py (10/10 with the existing host-default tests):

- Windows-layout fallback: Lib/site-packages with capital L.
- Multi-line MAPPING dict: locks in the [^}]* + re.DOTALL behaviour.
- SystemExit message contract: every actionable fix string and the
  attempted-paths list must appear; pins the user-facing recovery
  message so a future refactor doesn't drop a bullet.

End-to-end re-verified on this box: shadowing workspace_22/bin/unsloth
still serves 200 on / through the editable-finder fallback, with the
follow-up resolve-then-log change yielding [OK] Frontend loaded from
/mnt/disks/unslothai/ubuntu/unsloth/studio/frontend/dist.

Out of scope (called out by reviewers but deferred):

- _resolve_frontend_path candidate ordering still tries _PACKAGE_ROOT
  first. For the rare case where a shadowing install carries an older
  built dist, this serves the stale UI instead of the fresh one. Fix is
  non-trivial (the --local workflow intentionally wants _PACKAGE_ROOT to
  win when the cloned repo is the source of truth), so leaving it for a
  follow-up.
- studio/backend/colab.py still bails out on missing frontend instead of
  routing through the new resolver. Pre-existing behaviour, separate PR.
- _resolve_frontend_path is duplicated across run.py and unsloth_cli/
  commands/studio.py. Minor maintenance concern; consolidation is
  natural in a later refactor.

Addresses gemini-code-assist[bot] high-priority inline review on PR 5782
flagging that `mapping.get('studio')` could raise AttributeError if the
MAPPING regex matched a brace-delimited literal that ast.literal_eval
parsed as a non-dict (set, list, None). The regex `\{[^}]*\}` happily
matches `{1, 2, 3}` and literal_eval returns a set; the previous code
then crashed on .get().

Setuptools's editable-install template only emits dict literals so this
is defensive rather than a live bug, but the guard is one line per call
site and prevents a future template change from taking out backend
startup or CLI invocation.

Both call sites (studio/backend/run.py:558 and
unsloth_cli/commands/studio.py:234) now bail out on the finder file when
isinstance(mapping, dict) is False; the resolver keeps probing the
remaining finders, so a malformed entry in one finder cannot poison the
discovery of a good one elsewhere.

Adds test_resolver_does_not_crash_on_non_dict_mapping_literal to
test_frontend_resolution.py, which writes one bad finder (MAPPING is a
set literal) alongside one good finder (MAPPING is a real dict) and
asserts the resolver returns the good finder's dist path. Without the
guard this test crashes with AttributeError; with the guard it passes.

11/11 tests green.