fix(ui): comment editor textarea not focused on open in WebKit hosts

[BrandonNoad]

Problem

In Comment mode, selecting text (or double-clicking a word) opens the comment editor, but the textarea isn't focused — you have to tab several times or reach for the mouse before you can type. This happens consistently in WKWebView hosts (observed with the glimpseui integration on macOS); Chromium-based browsers are unaffected.

Root cause

CommentPopover focused the textarea from a mount effect keyed on [mode] that scheduled a setTimeout(0):

• In popover mode, the component renders null until position is computed by a later effect, so the textarea only mounts in the next commit.
• Whether the 0ms timer fires before or after that commit is engine-dependent task ordering. Chromium happens to commit the re-render first, so textareaRef.current is set when the timer fires. WebKit fires the timer first — the ref is still null, the effect never re-runs (mode doesn't change), and the editor opens unfocused.

Traced by instrumenting the page inside a real glimpse (WKWebView) window:

SCHED focus-timer d=0
FIRE focus-timer
DONE focus-timer active=BODY   ← callback ran, ref was null, focus() never called

Manually calling focus() on the same textarea afterwards worked fine, confirming the engine wasn't refusing focus — the app just never asked.

Fix

Focus from a ref callback on the textarea instead of a mount effect. The callback runs exactly when the element mounts, so there's no commit/timer race in either engine:

• Deferred (setTimeout(0)) focus and caret-to-end behavior are preserved.
• Popover ⇄ dialog switches still refocus, because the textarea remounts (this is what the old [mode] dep provided).
• isConnected guards the case where the popover unmounts before the deferred focus runs (also makes the StrictMode double-attach harmless).

Verification

• WKWebView (glimpse): instrumented driver over glimpse's JSON-lines protocol now logs focus(): TEXTAREA active=true and document.activeElement is the textarea as soon as the popover opens.
• Chromium: double-click-to-comment, drag-select-to-comment, and Expand (popover → dialog) all focus the textarea and accept typing immediately.
• tsc --noEmit on packages/ui/tsconfig.json and tsconfig.strict-consumer.json — clean.
• bun test packages/ui — 352 pass, 0 fail.