feat(ui): add Open In dropdown for diff files - #679

Add an "Open In" dropdown button to the after-pane (current view) header
in the diff viewer. Shows available editors that can open the file,
filtered to exclude terminals and Finder (directory-only openers).
- Add afterHeaderExtra snippet prop to DiffViewer for extensible headers
- Create OpenInDropdown component with click-to-toggle dropdown
- Pass worktreePath from BranchCard through DiffModal to construct
 full file paths for opening
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

- Add `kind` field to OpenerApp (editor/terminal/file-browser) so the
 frontend filters by kind instead of maintaining a hardcoded blocklist
- Use position:fixed with getBoundingClientRect() to prevent the
 dropdown menu from being clipped by overflow:hidden on diff panes
- Add .catch() handler on getAvailableOpeners() to log failures
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

When opening a file from the diff viewer or branch actions bar, editors
that support it (VS Code, Cursor, Zed) now receive the worktree path as
a project argument via `open -b <bundle_id> --args <project> <file>`.
This opens the file within the worktree workspace context, enabling
search, go-to-definition, and git integration scoped to the worktree.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Replace `open -b <bundle_id> --args` with dedicated CLI tools (`code`,
`cursor`, `zed`) that properly handle window reuse, IPC, and workspace
resolution. Falls back to `open -b` without `--args` when the CLI is
not installed or for other apps.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>