feat(chat): let the composer be dragged taller

[luoxuanzao]

Summary

• Bring back the drag-to-resize composer from the reverted fix(chat): refine compact controls, settings, and update cues #56, in isolation. A thin grip above the message input lets the user drag to set the input height (clamped between 140px and 75% of the chat view), step it with ArrowUp/ArrowDown (24px), and return to the automatic content-driven height via double-click or Home. The grip is a focusable role=separator with aria value bounds, and re-clamps on container resize.
• Why: long prompts kept clipping at the default fixed box height, with no way to enlarge the input.

Verification

• npm run typecheck
• npm run lint
• npm run test — 161 suites / 3492 tests pass, incl. composer-resize.test.ts
• npm run build — covered by CI on this PR
• npm run release:check — covered by CI on this PR
• npm run audit:prod — covered by CI on this PR
• Tested affected behavior live: built to the dev vault and exercised the grip in Obsidian (ArrowUp ×2 → 140→188px with --resized class + height var; Home → reset to 140px)

Safety

• No credentials, private vault content, internal URLs, or personal paths are included
• User-visible changes are documented in CHANGELOG.md (added under [Unreleased])

Co-authored-by: QoderAI (Qwen 3.8 Max) qoder_ai@qoder.com