fix: stabilize composer typing state

[ronak-guliani]

Summary

Polishes the chat composer with upstream editing fixes and an opt-in glass visual treatment.

• Adds chip-aware selection painting, macOS Home/End navigation, and single-pass mention paste handling.
• Improves trait menus with explicit blue selection checks and default badges.
• Applies the translucent composer surface only when sidebar translucency is enabled and reduced motion is not requested.

Problem

Inline composer chips were omitted from native text-selection painting, macOS Home/End navigation could be inconsistent around decorator nodes, and copied mentions could fall through to plain-text paste behavior. The composer also lacked the upstream translucent surface and clearer trait selection treatment.

Solution

Port the isolated Lexical editor primitives and their regression coverage from upstream. Keep the glass surface performance-safe by preserving the opaque default and activating backdrop blur only for users who already opt into sidebar translucency and do not request reduced motion.

[ronak-guliani]

Review outcome: no high-confidence implementation defect was found, but validation and regression coverage are incomplete.

Please add a focused test for IME composition around the ComposerPromptEditor change, proving composition input is not interrupted or committed prematurely while typing state updates. Then rerun pnpm fmt:check, pnpm lint, pnpm typecheck, and pnpm test.

The Windows Smoke failure appears to be CI setup (pnpm unavailable), but the main CI run was cancelled/stale, so a completed validation run is still required before merge.

[ronak-guliani]

Added focused browser coverage for controlled IME input in ComposerPromptEditor. The test verifies that Enter during composition does not invoke command/submission handling, does not bubble, and preserves the composed draft through the controlled state update and composition end.

Validation completed:

• pnpm fmt:check
• pnpm lint
• pnpm typecheck
• pnpm test
• focused Chromium browser test for ComposerPromptEditor.browser.tsx