fix(gitlab): persist unposted inline comments + split browser-safe types

[backnotprop]

Summary

Closes #680.

• submitGlMRReview (packages/shared/pr-gitlab.ts): when one or more inline discussion POSTs fail (e.g. transient i/o timeout against GitLab), the failed comment bodies are now persisted to ~/.plannotator/failed-comments/{host}-{project}-mr{iid}-{ts}.json so they can be recovered. The error message points the user at the file path.
• The original throw-vs-warn split is preserved deliberately and now commented in the source:
  • All inline comments failed → throw. Nothing was posted, so the UI retries from a clean state.
  • Partial failure → warn only. The MR note and the discussions that did succeed are already on GitLab; throwing would have the client resubmit the whole review and create duplicates.

Why the diff is bigger than #680 alone

The persistence call needs fs/os/path. The review-editor browser bundle was pulling pr-gitlab.ts in as dead code through pr-provider.ts's static imports, which made the file silently constrained to "no Node-only imports." Adding fs broke bun run --cwd apps/review build.

So I split pr-provider.ts along its existing seam:

• pr-types.ts (new) — types, label helpers (getPlatformLabel, getMRLabel, ...), parsePRUrl. No imports of pr-github / pr-gitlab. Browser-safe by construction.
• pr-provider.ts — server-only dispatch (submitPRReview, fetchPR, ...). Re-exports pr-types so existing server-side imports keep working unchanged.
• Browser code in packages/review-editor/* now imports from pr-types (~14 files, all type-only or pure-helper imports).

Test plan

• bun test packages/shared/ — 272 pass, 0 fail
• tsc --noEmit on packages/shared, packages/ai, packages/server, packages/ui, apps/pi-extension — all clean
• bun run --cwd apps/review build — review browser bundle no longer fails on __vite-browser-external
• bun run build:hook — full hook build green
• bun build apps/hook/server/index.ts --compile — single-file binary builds (107 MB output)
• vendor.sh updated to vendor pr-types.ts into apps/pi-extension/generated/; Pi typecheck clean
• Manual: trigger a GitLab MR review against a flaky network and confirm ~/.plannotator/failed-comments/*.json contains the unposted bodies, while already-posted comments are not duplicated on retry