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

[binbandit]

Summary

Adds multi-select support for sidebar thread items, enabling bulk actions on threads.

• Cmd/Ctrl+Click toggles individual thread selection
• Shift+Click range-selects from the last anchor to the clicked thread (within the same project)
• Right-click on selection shows a bulk context menu with "Mark unread (N)" and "Delete (N)"
• Escape or plain click clears the selection
• Selected threads share the same highlight style as the active/navigated thread

Implementation

New: threadSelectionStore.ts

Ephemeral Zustand store (no persistence — selection clears on reload) managing:

• selectedThreadIds: ReadonlySet<ThreadId> — the selected set
• anchorThreadId — stable anchor for shift-click range computation
• Actions: toggleThread, rangeSelectTo, clearSelection, removeFromSelection
• No-op guards to avoid unnecessary re-renders

Refactored: Sidebar.tsx

• Extracted deleteThread helper from handleThreadContextMenu for reuse by bulk delete. Accepts an optional deletedThreadIds set so bulk delete correctly finds fallback navigation targets.
• Platform-aware modifier detection — uses isMacPlatform(navigator.platform) (same pattern as terminal-links.ts) instead of the naive metaKey || ctrlKey approach, so Ctrl+Click on Mac doesn't conflict with context menu.
• Context menu routing — right-clicking a selected thread shows the bulk menu; right-clicking an unselected thread clears the selection and shows the normal single-thread menu.
• Escape key integrated into the existing keydown effect.

New: threadSelectionStore.test.ts

22 unit tests covering:

• Toggle add/remove/preserve behavior
• Anchor management (set on add, preserve on deselect, clear on removal)
• Range selection: forward, backward, anchor stability, cross-project fallback, same-thread range
• clearSelection no-op guard (referential stability)
• removeFromSelection with anchor cleanup and no-op when nothing matches

Video

Screen.Recording.2026-03-10.at.11.50.03.am.mov

Verification

• bun lint — 0 errors (2 pre-existing warnings in server package, unrelated)
• bun typecheck — 7/7 packages pass
• bun run test — all 22 new tests pass; 27 pre-existing failures in composerDraftStore.test.ts and terminalStateStore.test.ts (localStorage unavailability in test env, confirmed identical before this change)

Note

Add shift-click and cmd/ctrl-click multi-select for sidebar threads

• Introduces a new Zustand store (threadSelectionStore.ts) tracking selected thread IDs and an anchor thread for range selection.
• Cmd/Ctrl+Click toggles individual thread selection; Shift+Click range-selects within the current project; plain click clears selection and navigates normally.
• When multiple threads are selected, right-clicking opens a bulk context menu supporting mark-unread and delete for all selected threads.
• Pressing Escape or clicking outside thread items clears the active selection; project header clicks also clear selection.
• Selected threads are visually distinguished from the active thread in the sidebar render.

^{Macroscope summarized 970066c.}

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[juliusmarminge]

video pls

[juliusmarminge]

shift click to me usually does range selection, but this one works the same as cmd clicking by just selecting the one you click?

also it uses the same style as the currently active thread so looks like i have more selected than it actually is:

CleanShot.2026-03-09.at.17.32.22.mp4

[binbandit]

shift click to me usually does range selection, but this one works the same as cmd clicking by just selecting the one you click?

also it uses the same style as the currently active thread so looks like i have more selected than it actually is:

CleanShot.2026-03-09.at.17.32.22.mp4

I used the same color, so as to not introduce another design style. Can change. Got a color you would like me to use?
As for the range selection, yes that was the intention. It did work, but appears it no longer works. Let me debug and update ☺️

P.S. If you reach out to me on x: @0x1f99d -- It would be good if we can arrange a better form of communication (eg slack). So we can communicate easier on these

[binbandit]

Here is the latest video.

Screen.Recording.2026-03-10.at.11.50.03.am.mov

Fixed the range select, also added a nice little blue highlight color.

@juliusmarminge