Swap the default system allocator for mimalloc (Linux-gated) and tune the
release profile for runtime performance: fat LTO, single codegen unit, and
opt-level 3.

#22)

* fix: add gnome-screenshot fallback for background screenshot capture

The screenshot tool failed from background processes (systemd user
services, non-interactive parent shells) on GNOME Wayland. Both existing
capture rungs are structurally unavailable in that context:

- GNOME Shell's `org.gnome.Shell.Screenshot` rejects callers that do not
  own an allowlisted bus name (DBusSenderChecker), so the direct DBus call
  returns ACCESS_DENIED.
- The XDG portal cancels with response code 2 when there is no foreground
  window to parent its (non-interactive) request to.

Add `gnome-screenshot` as a third fallback rung: it claims an allowlisted
bus name and works regardless of session context. The subprocess is bounded
by a 20s timeout (matching the portal) and the child is killed + temp file
cleaned on timeout, so a hung capture degrades to a clear error instead of
blocking the tool forever.

Also add `COMPUTER_USE_LINUX_SCREENSHOT_BACKEND` to force a single backend
(gnome-shell | portal | gnome-screenshot), skipping the fallback chain for
pinned/background deployments and debugging. Diagnostics now probe
gnome-screenshot and surface it in capabilities.screenshot when present.

Fixes #20.

* refactor: use tokio::process for gnome-screenshot capture

Address review feedback: replace the manual try_wait polling loop and the
hand-rolled PATH lookup with tokio::process::Command. It searches PATH
natively when spawning and provides a real async wait, removing the 50ms
poll latency and CPU spin. Bound the wait with tokio::time::timeout and
kill the child on deadline. Enables the tokio "process" feature.