fix(chat): keep hover dropdowns inside the input toolbar

[luoxuanzao]

Summary

• The external-context and MCP hover dropdowns are centered on their toolbar icons with a fixed width. When the icon sits near the left edge of the toolbar, the centered panel overflowed the chat container's clipped edge and lost its leading characters — the panel title and the ~ path prefixes were cut off.
• Adds a pure placement module (hover-dropdown-placement.ts) that clamps the panel center so it stays inside the input toolbar with an 8px edge inset, and caps the width only when the toolbar itself is narrower than the panel.
• Both selectors publish the result as CSS custom properties on hover and on dropdown render; the previous static values remain as CSS fallbacks.
• Fixes the clipping in both external-context and mcp-selector panels (same centered pattern).

Verification

• npm run typecheck
• npm run lint
• npm run test
• npm run build
• npm run release:check
• npm run audit:prod
• Tested affected qodercli behavior against a real CLI when applicable

Verified on a live Obsidian vault over CDP in two scenarios: at full sidebar width the 320px panel renders fully inside the container, and with a 300px-wide toolbar the panel shrinks to 258px with the title and ~ prefixes intact. 13 new unit tests cover the placement math; full suite (164 suites / 3523 tests) passes.

Safety

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