fix: keep composer mention menu with the caret

[wibus-wee]

Related issue

None. This is an internal composer mention-placement regression.

Problem

Anchoring the main chat menu to the composer frame disconnected it from the caret. The floating positioner could then place a tall menu below the caret even when several rows fit above. Its inline viewport-sized max-width also overrode the input-width cap, stretching long command descriptions off the right edge.

Summary

• Keep the desktop main composer menu on the caret and above it while a heading and one option fit; cap its height and scroll the list instead of flipping for the full menu height.
• Retain horizontal collision adjustment and the input-width cap. At the true top edge, fall below only when no useful row fits above.
• Add wide-desktop and 600px-high desktop stress regressions and update the bilingual Spec and decision note.

Verification

• Playwright: 6 passed. The new 800×600 test fails before the vertical fix (menu bottom y≈593 below the caret), then passes with the tall list above, scrolling, filtering, and backspacing without a side jump. Other cases cover the 2048×1098 width regression, caret movement, resize and scale, top-edge fallback, and inline-editor focus.
• Related component Vitest: 46 passed. Component typecheck, changed-file oxlint, formatting, and docs check passed in an isolated checkout.
• Full pnpm check passed typecheck and lint, then failed in unchanged CLI workspace-git-service.test.ts (1 failure among 3245 tests). No CLI files changed.
• Wide before/after screenshots and 800×600 tall/filtered after screenshots were uploaded to the Lody conversation. An earlier 650×250 screenshot was not a valid desktop acceptance case because the window minimum height is 600px; these screenshots supersede it.

Context handoff

Internal same-repository branch; external contribution handoff is not applicable.