Replace the composer's JS autosize with native field-sizing

[dovvnloading]

Problem

The composer textarea sized itself with a one-shot layout effect: reset height to auto, read scrollHeight, clamp to 42-160px, re-running only when the draft text changed. A scrollHeight read is only as good as the layout at the instant it runs, so any mount under a transient layout - a viewport resized after load, a zoom or DPI change - baked a wrong height into the inline style, and nothing corrected it until the next keystroke. In the failure mode this fix was traced from, an empty one-row composer rendered pinned at the 160px ceiling.

Change

field-sizing: content on .composer-input, with min-height: 42px / max-height: 160px / overflow-y: auto. The height now tracks the content continuously instead of being sampled once, so it cannot go stale. Textareas are border-box in Chromium's UA sheet, so the bounds match the old JS 42-160 range exactly; this is the same native-autosize posture the code-sandbox card's inputs already use. The layout effect, the ref that existed only for it, and the now-unused useLayoutEffect/useRef imports are removed; the skip-link jump target is the element id and is unchanged.

Test plan

• Composer.test.tsx: 38 pass unchanged.
• npm run check (schema drift, typecheck, lint, vitest, build, bundle size) against a clean checkout of this branch: 2135 pass.
• Manual against the built bundle at the app's real 1440x900 window: empty composer renders at the same geometry as before the change (textarea 50px, dock 114px, no inline height); a four-line draft grows it to ~81px and clearing returns it to 50px; two live viewport resizes with no reload leave it at 50px, the exact sequence that previously left the old implementation stuck at 160px.