* feat(web): add shift-click multi-select for sidebar threads

Add support for selecting multiple threads in the sidebar via
Cmd/Ctrl+Click (toggle) and Shift+Click (range select), with
bulk context menu actions for Delete and Mark Unread.

- New threadSelectionStore (Zustand) for ephemeral selection state
  with anchor-based range selection and no-op guards
- Platform-aware modifier detection (Cmd on Mac, Ctrl elsewhere)
  matching existing isMacPlatform pattern in terminal-links
- Extracted deleteThread helper from handleThreadContextMenu for
  reuse by bulk delete, with deletedThreadIds set for correct
  fallback navigation during batch operations
- Selected threads share the active thread highlight style
- Escape key clears selection; plain click clears and navigates
- Right-click on selected thread shows bulk menu; right-click on
  unselected thread clears selection and shows single-thread menu
- 22 unit tests covering toggle, range select, clear, remove,
  and edge cases (cross-project fallback, anchor stability)

* fix(web): filter deleted threads from worktree orphan check during bulk delete

When bulk-deleting threads that share a worktree, the stale threads
closure caused getOrphanedWorktreePathForThread to see the other
threads-being-deleted as still alive, suppressing the orphaned
worktree cleanup prompt. Filter out deletedThreadIds (keeping the
current thread) so the check correctly identifies worktrees that
will have no surviving references.

* fix(web): fix shift-click range selection, distinct selection styling, and deselect on click away

* Preserve thread multi-select on safe sidebar control clicks

- avoid clearing selection on mousedown for thread items and marked safe controls
- centralize the selection-safe selector and add logic tests for clear vs preserve behavior
- simplify thread selection store typing by merging state/actions interface

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Co-authored-by: Julius Marminge <julius0216@outlook.com>