fix(composer): flush debounced autocomplete detection on Tab/Enter

[wesbillman]

Problem

Reported by DK: typing @cl and quickly hitting Tab right after the composer regains focus commits "@fizz" (the first entry of the unfiltered list) instead of the intended match, forcing him to back it out and retype.

Root cause

Composer autocomplete detection is debounced 120 ms (MENTION_DEBOUNCE_MS, introduced in #222 to reduce input lag): keystrokes only stash the latest text in refs and schedule setMentionQuery for later. But handleMentionKeyDown commits Tab/Enter synchronously against suggestions[selectedIndex] — a list ranked for the previous query. Type inside the debounce window and Tab commits the stale list's top entry. The #channel and :emoji: autocompletes share the identical pattern.

Fix

On Tab/Enter, if a detection is pending, flush it synchronously:

• Mentions / channel links — re-run detection on the latest text (already in refs) and, when the query changed, re-rank synchronously and commit the fresh top match. A single fast Tab now inserts what you actually typed. If the fresh query matches nothing, the keypress is swallowed instead of committing garbage.
• Emoji — results come from an async search, so a stale Tab refreshes the dropdown for the new query instead of committing a wrong pick.
• All three hooks also move the insert start-index from state to a ref, so the replaced text range can't lag behind the committed query either.

useMentions.ts crossed the 1000-line guard with the flush branch, so the pure candidate helpers (MentionCandidate type, label/owner formatters) moved to a new mentionCandidates.ts — no logic changes there.

Testing

• New Playwright regression test: open dropdown with bare @, type bo + Tab inside the debounce window, assert @bob is inserted. Verified it fails against the unpatched code and passes with the fix.
• Full mentions.spec.ts e2e suite: 31/31 passing.
• pnpm test (1625 unit tests), pnpm typecheck, pnpm check (biome + file sizes) all green.

Fixes the race DK reported in typing-improvements.