…d frame

AstroHan W1 second retest flagged 3 structural regressions on the W1
leaves; this commit fixes the trio in one atomic change because all
three are CSS-only.

1. **Text selection regression (#1)** — The app shell sets `select-none`
   on the timeline body (layout.tsx L2320) and opts back in for input,
   textarea, and `[contenteditable]` only. The legacy carve-out at
   message-part.css L710 named `[data-component="user-message"]
   [data-slot="user-message-text"]` and `[data-component="text-part"]`
   so legacy bubble + assistant prose stayed selectable; the W1 leaves
   replaced both data-components without an equivalent carve-out, so
   bubble text and agent prose stopped responding to drag-select.
   Reinstated by adding `user-select: text` + `cursor: text` to
   `[data-slot="bubble-text"]`, `[data-slot="agent-prose"]`, and
   `[data-slot="agent-reasoning"]`.

2. **Trow chev direction (#3)** — The W1 icon is `chevron-down` (rest
   reads "click to expand"), but the open-state rotation was 90°,
   which pointed the glyph sideways. Per AstroHan's flag the open
   chev should read "click to fold up" — 180° rotates `chevron-down`
   to chevron-up, restoring the disclosure idiom.

3. **Trow expanded nested frame (#4)** — DESIGN.md L417 scopes the
   transparent + 1px `--border-weaker` + radius-sm body frame to each
   per-tool result body, not the trow group wrapper. The W1 trow-block
   wrapped the inner sub-rows in its own frame, which composed with
   the legacy per-tool chrome and produced a visible double border.
   Dropped the wrapper's border + padding so it only owns the vertical
   rhythm between sub-rows; the per-tool body keeps its own frame.

Tests still pass: `session-turn-user-bubble`, `session-turn-agent-round`,
and `session-turn-trow-block.reducer` suites are CSS-grep + reducer
based and do not assert any of the changed declarations.

… WebFetch parity

AstroHan's fourth W1 retest (msg=ac13481a) flagged three trow visuals.

**#1 — Missing expand/collapse animation.** The header comment on
`session-turn-trow-block.css` claimed `session-turn.css` owned the
`::details-content` rule, but it was never actually wired anywhere.
Add the rule (mirror of preview `message-flow.html` L13-23): height +
opacity transition gated by `prefers-reduced-motion: no-preference`,
with `transition-behavior: allow-discrete` covering the discrete
`content-visibility` step. The `:root { interpolate-size:
allow-keywords }` declaration in utilities.css is what lets
`height: 0` ↔ `height: auto` interpolate; the rule silently no-ops to
a snap without it. Inner Collapsible.Content gets a matching scoped
animation — Solid's `collapsible.css` ships its keyframes commented
out (L92-101), so without the override the per-tool body would snap
open. The scoped selector keeps the file-tree / settings Collapsibles
on their default behaviour.

**#2 — Tighter expanded body.** AstroHan reported the expanded trow
body felt too loose vs preview. `.mf-tlist` in
`docs/design/preview/message-flow.html` L199 locks gap at 6px between
sub-tool rows; trow-body now matches (was 8px).

**#3 — WebFetch layout parity.** WebFetch is the only tool that ships
a right-aligned `[data-component="tool-action"]` arrow inside its
trigger row (`message-part-tools-basic.tsx` L142). The other tools
use the plain `trigger={{ title, subtitle }}` object form, so they
already flow left without the action chrome. Suppress the action icon
only inside the trow result body — standalone WebFetch outside the
trow keeps the affordance.

…resize sync (#887)

Three connected fixes for the right-panel titlebar seam, reported post-release as a follow-up to #880. All three only become visible together: once the seam aligns, the toggle slide and the drag-resize lag become readable.

**Fix 1 — macOS seam alignment (8px)**
`padding-inline: 8px` → `padding-inline-start: 8px` on the macOS `[data-component="titlebar-shell"]` rule (`packages/app/src/index.css`). The symmetric 8px padding pushed the right rail 8px inboard of the viewport while the `<aside>` directly below reached the actual edge — `border-l`s ended up 8px apart on macOS only. Windows already correct.

**Fix 2 — Stable toggle position across open/close**
`#pawwork-titlebar-right` (`packages/app/src/components/titlebar.tsx`) changed from in-flow flex sibling of the tabs slot to absolute-positioned at the rail's top-right corner (`right-2` macOS / `right-0` Windows). PR #880's flex layout slid the toggle by `--right-panel-width` on a 240ms transition; that motion read as jarring once fix #1 aligned the seam.

**Fix 3 — Drag-resize seam sync**
New effect in `SessionSidePanel` mirrors `props.size.active()` to a `data-resizing-right-panel` attribute on `<desktop-shell>`; matching CSS rule in `index.css` disables the shell's transition while the attribute is present. During drag the body's inline `width` snapped per-pointermove while the shell's `--right-panel-width` kept its 240ms transition — the two sides of the seam moved at different speeds. Attribute-bridge avoids plumbing session-scope sizing state into the global layout context.

**Hardening from code review**
- P1 (toggle/+ collision risk): added `padding-right: 44px` reserve on `#pawwork-titlebar-tabs` when the rail is active (toggle 30 + `right-2` 8 + gap 4 + 2px buffer for Tabs.List `px-1`) so the no-collision contract no longer depends on Kobalte's Tabs.List rendering at content-width.
- P2 (attribute leak): effect now unconditionally `removeAttribute` first then re-evaluates, plus `onCleanup` clear. Closes leaks via `isDesktop()` flip, component unmount, and stale residue.
- P3 (test quality): tightened tolerance to 1px, replaced `waitForTimeout` with `expect.poll`, added `+` click test and a collision-guard that runtime-injects `width: 100% !important` on Tabs.List.
- Bot-review follow-up: x-axis seam test now polls for steady state (panel open/close runs a 240ms transition that the drag-only gate doesn't suppress); toggle test normalizes `aria-expanded` to false before sampling closed state.

**Regression tests** (`packages/app/e2e/session/titlebar-right-rail-contract.spec.ts`, 8 total)
- `border-l aligns horizontally with right-panel body` — pre-fix #1: `Received: 8`; post: passes
- `toggle keeps the same x-position across open/close` — pre-fix #2: `Received: 229.25`; post: <1px
- `+ add-tab button stays clickable and does not close the panel`
- `+ stays clickable even if Tabs.List is forced to fill the slot (collision guard)`
- `data-resizing-right-panel attribute does not leak when viewport drops below the desktop breakpoint mid-resize state`
- Plus 3 pre-existing tabs-rail contract tests

**Verification**
- `bun run test:e2e:local titlebar-right-rail-contract.spec.ts` → 8/8 pass
- `bun run snap right-panel-titlebar` → pass, no visual regression
- `bun run lint` → clean
- `bun --cwd packages/app run typecheck` → clean
- Manual Electron (macOS): toggle stable across open/close, drag-resize seam moves as one continuous line
- CI: 27/27 green

**Follow-up tracked separately**
- #889 — `session-side-panel.tsx` is 604 lines and growing; refactor scheduled in a dedicated PR.

**Risk notes**
- Fix #1 macOS-only; fixes #2 and #3 affect both platforms.
- Closed-state toggle inset changes from 16px to 8px on macOS, matching native toolbar conventions.
- Open-state 240ms toggle slide is removed; body width still animates as before.
- No `.github/workflows/` touched.