feat(composer): keep the resize grip out of the way

[luoxuanzao]

Summary

• The resize grip above the message input no longer shows a hover tooltip,
  and it stays hidden until the pointer is inside the input area or the
  composer has focus. The input's top edge therefore reads as a plain border
  while you are reading or scrolling, matching the Qoder app.
• Hovering the grip or dragging it still shows the accent highlight, and
  drag, double-click to reset, and keyboard resizing are unchanged.
• Documented under [Unreleased] in CHANGELOG.md (Changed + Removed).

Verification

• npm run typecheck
• npm run lint
• npm run test (166 suites, 3555 tests)
• npm run build
• npm run release:check
• npm run audit:prod
• Tested affected qodercli behavior against a real CLI when applicable —
  no CLI behavior is involved in this change; the grip states (hidden,
  revealed on hover, held while focused, accent while dragging) were checked
  against the built styles.css in Chromium computed styles.

Safety

• No credentials, private vault content, internal URLs, or personal paths are included
• User-visible changes are documented in CHANGELOG.md