feat: split worktree/diff-type into separate controls (#248)

* feat: split worktree/diff-type into separate controls

The single dropdown that mixed diff type selection (uncommitted, last-commit,
branch) with worktree switching was confusing — entering a worktree silently
replaced all dropdown options with no persistent indicator of which worktree
was active.

Split into two controls:
- Context dropdown: switches between main repo and worktrees (only shown
  when worktrees exist, highlighted when a worktree is active)
- View dropdown: always shows the same diff type options regardless of context

Server changes:
- GitContext now exposes worktrees as a separate field
- Removed getWorktreeDiffOptions() and back-to-main sentinel handling
- Simplified /api/diff/switch (no more diffOptions replacement)

Client changes:
- New activeWorktreePath state + handleWorktreeSwitch callback
- handleDiffSwitch composes worktree prefix from context automatically
- Derived activeDiffBase strips worktree prefix for dropdown display

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor: derive activeWorktreePath from diffType instead of separate state

Eliminates redundant state — activeWorktreePath and activeDiffBase are
now both derived in a single useMemo from the composite diffType string.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor: move DiffOption, WorktreeInfo, GitContext to @plannotator/shared

Eliminates duplicate interface definitions across server and client.
Types now live in packages/shared/types.ts and are imported by both
packages/server/git.ts and packages/review-editor/.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

packages/review-editor/package.json

@@ -7,6 +7,7 @@
     "./styles": "./index.css"
   },
   "dependencies": {
+    "@plannotator/shared": "workspace:*",
     "@plannotator/ui": "workspace:*",
     "@pierre/diffs": "^1.1.0-beta.19",
     "highlight.js": "^11.11.1",