refactor(web): typing no longer re-renders the composer model picker#22721

Every keystroke in the composer re-rendered the model picker and the attach button, even though neither depends on the prompt.

The fix uses component boundaries, not stable-callback wrappers. This adds no useStableCallback uses; ChatComposer keeps the 34 it has on main.

• ComposerModelPicker (in ComposerFooterControls.tsx) wraps ProviderModelPicker. It defines the single-model and multi-model toggle handlers itself, from the multi-model selections and their setter, selectedModelSelection, the instance entries, the model options and onProviderModelSelect. None of those change while you type, so the compiler keeps the handlers. ChatComposer builds the <ComposerModelPicker> element as its own const. The controls row around it is rebuilt on every keystroke because TraitsPicker takes prompt, but the picker element itself stays cached.
• ComposerAttachButton owns the hidden file input and builds its change handler from addComposerAttachments and focusComposer.

Renders measured with the shared fiber counter (rc-init.js) on a seeded dev server: typing 20 characters into an existing thread's composer, median of 3 runs.

Still rendering on each keystroke:

• ComposerAttachButton itself. addComposerAttachments gets a new identity on every keystroke because of the prompt-editing helpers it calls. Its tooltip, button and icon children no longer re-render.
• TraitsPicker. It takes prompt to detect "ultrathink", which needs a change to TraitsPicker itself. Both are left for follow-ups.

Tests run: vp lint and vp fmt --check on the changed files, tsc-rs --noEmit for apps/web, and reactCompilerCoverage.test.ts for the changed files (0 bailouts).

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