feat: copy popover, optimistic sub-chats, worktree cleanup, sortable sidebar, draggable tabs

[aletc1]

Summary

Bundles six related UX/cleanup improvements plus a merge of dev:

• F1 Copy button in the text-selection popover
• F2 Optimistic sub-chat creation (instant UI, RPC in background, rollback on failure)
• F3 Worktree directory cleanup (rmdir guard, project-delete cascade, startup orphan scan)
• F4 Native HTML5 DnD on tab bar + grab cursor
• F5 Sortable sub-chats sidebar via @dnd-kit + grip handle on hover
• F6 Cached file_stats_* columns on sub_chats + auto-delete empty sub-chats on tab close / app quit

Test plan

F1 — Copy button in selection popover

• Select text in an assistant message → popover shows Add to context + Copy. Click Copy → CopyIcon morphs into CheckIcon for ~2s. Paste in another app → matches.
• Select text inside a diff or tool-edit block → popover shows Add to context + Copy + Reply. Copy still works.
• Selection in file-viewer → popover does not appear (uses native context menu).

F2 — Optimistic sub-chat creation

• Local mode: click + 5× rapidly → all 5 tabs appear instantly with no waiting spinner.
• Reload window → all 5 sub-chats persisted (DB write succeeded in background).
• Sandbox mode: + still uses lazy creation; first message persists the row.
• Failure path: temporarily throw inside chats.createSubChat server-side → tab appears, then disappears with toast "Failed to create chat".

F3 — Worktree cleanup

• Create a chat → ~/.21st/worktrees/<slug>/<folder>/ exists. Delete the chat → directory is gone.
• Create 3 chats in one project, then delete the project → all 3 worktree dirs gone, parent slug dir gone if empty.
• Pre-stage an orphan: mkdir ~/.21st/worktrees/fake/orphan && touch ~/.21st/worktrees/fake/orphan/x. Restart the app → log line [WorktreeCleanup] Scanned N worktree dirs, removed M orphans shows the orphan was removed (~5s after start).
• Tamper test: in db:studio, set a chat's worktreePath to /tmp/test. Delete the chat → /tmp/test is not removed (path-prefix guard rejects).
• Symlink test: ln -s ~ ~/.21st/worktrees/<slug>/<folder>/escape. Delete the chat → home dir contents intact (Node fs.rm only unlinks the symlink, doesn't follow it).

F4 — Native DnD on tab bar

• Switch to tabs mode (close the Chats sidebar via the « icon). Open ≥ 2 sub-chats.
• Hover a tab → cursor shows grab. Hover a split-pair tab (with border) → cursor stays pointer (drag disabled).
• Click+drag a tab horizontally → insertion marker appears on left/right edge of hovered tab. Release → tabs reorder.
• Reload window → order persisted via openSubChatIds localStorage.
• While dragging, a tab being dragged shows reduced opacity.

F5 — Sortable sidebar with @dnd-kit

• Switch to sidebar mode. Hover a row → grip icon (⋮⋮) fades in on the left edge; cursor shows grab.
• Drag a row up/down within its section → reorders. Reload → persisted.
• Pinned and unpinned sections sort independently — dragging across sections is not supported.
• Split-pair rows (with border) → grip icon is hidden, cursor is pointer (drag disabled).
• Single click without drag still switches the chat (4px activation distance).

F6 — File stats cache + auto-delete empty

• Create a sub-chat with no messages → close its tab → row is deleted from DB (verify via bun run db:studio).
• Create a sub-chat → send a message → close tab → row preserved.
• Create a sub-chat → quit app via Cmd+Q → reopen → empty unnamed sub-chats are gone, named/messaged ones survive.
• File-stats numbers (additions/deletions/file count) in the sidebar match what they were pre-change.
• Make a new edit via Claude → sidebar stats update on next refresh.
• Streaming-in-flight: close a tab while it's mid-stream → final write still lands (delete is gated on useStreamingStatusStore.isStreaming(id)).

Regression smoke

• bun run dev boots without console errors.
• Existing context-menu actions on sub-chats still work (rename, archive, pin, split).
• Multi-window: open a 2nd window, create a chat there — no cross-window deletion.
• No new TypeScript errors: bun run ts:check (or npx tsc --noEmit) is no worse than baseline.

Migration

A new Drizzle migration drizzle/0008_shiny_hydra.sql adds three columns to sub_chats. Auto-runs on app start; no manual step needed.