Focus diff comment popover from ref

[nwparker]

Summary

• Move the diff-comment popover textarea mount focus from a passive Effect into the textarea ref callback.
• Keep the document mousedown outside-click subscription in an Effect because that is real external DOM synchronization.
• Removes 1 Effect hook call site from DiffCommentPopover.tsx.

Validation

• pnpm exec oxlint src/renderer/src/components/diff-comments/DiffCommentPopover.tsx
• pnpm exec vitest run --config config/vitest.config.ts src/renderer/src/components/diff-comments/diff-comment-popover-position.test.ts
• pnpm run typecheck:web
• git diff --check
• Effect hook AST count: origin/main 911, working 910

Risk

Medium. This changes editor/review-note popover focus timing only; it does not change note persistence, diff comment submission, IPC, SSH, runtime ownership, or git-provider behavior.

Risk assessment

Risk: MEDIUM

Historic risk metadata backfill based on the existing risk:medium label.

Risk: medium

Historic risk metadata backfill based on the existing risk:medium label.

[nwparker]

Risk-down blocked: exact head is not mergeable against current origin/main, so I am leaving this PR at risk:medium.

Exact head: 72c4761c79bbb77ae7c41c0d0d42e8a63cdbf28c

Mergeability evidence:

• git diff --check origin/main...refs/riskdown/pr-3333 passed.
• git merge-tree --write-tree origin/main refs/riskdown/pr-3333 reports a content conflict in package.json.
• The merge also auto-merged the touched renderer files, but exact-head validation is blocked by the package manifest conflict.

Because the exact PR head cannot be cleanly validated or merged on top of current main, I did not lower the risk label.

[nwparker]

Validated and merged.

Evidence:

• Rebased and pushed final head 2ef3f87de493e89d92cc9f1dc2a096dff9def1c5.
• GitHub PR Checks verify succeeded on that head: https://github.com/stablyai/orca/actions/runs/26710300115/job/78719174287
• Local checks passed: diff-comment-popover-position.test.ts, pnpm run typecheck:web, targeted oxlint, targeted oxfmt --check, and git diff --check origin/main...HEAD.
• Merged with rebase/admin. Merge commit: f24e987578761e1ec98778932fc6fadc7898bc53.