feat(desktop): conversation timeline rail for long threads

[OutThisLife]

Summary

Screen.Recording.2026-06-22.at.18.19.54.mov

• Adds a right-edge conversation timeline in the desktop chat: one dither tick per user prompt (≥4 turns), hover popover with truncated previews, click to smooth-scroll back
• Active tick tracks scroll position; popover stays open while hovering (no close-on-click)
• Inset from pane hover-reveal strip (HOVER_REVEAL_MAIN_EDGE_CLEARANCE) so collapsed sidebar/file-browser rails don't steal hover
• Matches existing GUI: composerPanelCard / tool font, hover:transition-none, session-row hover timing, cursor-pointer

Test plan

• Thread with ≥4 user prompts shows ticks on the right; shorter threads hide the rail
• Hover ticks → popover lists truncated one-line previews; move between rows without popover closing
• Click row/tick → scrolls to that user bubble; active tick updates on scroll
• Collapsed left/right sidebar: hovering timeline does not peek-reveal adjacent panes
• npx vitest run apps/desktop/src/components/assistant-ui/thread-timeline-data.test.ts

[github-actions]

🔎 Lint report: bb/desktop-thread-timeline vs origin/main

ruff

Total: 0 on HEAD, 0 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 0 pre-existing issues carried over.

ty (type checker)

Total: 11463 on HEAD, 11461 on base (🆕 +2)

🆕 New issues (2):

Rule	Count
unresolved-attribute	2

First entries

tests/run_agent/test_credits_notices_toggle.py:76: [unresolved-attribute] unresolved-attribute: Unresolved attribute `_credits_session_start_micros` on type `AIAgent`
run_agent.py:2984: [unresolved-attribute] unresolved-attribute: Object of type `Self@get_credits_spent_micros` has no attribute `_credits_session_start_micros`

✅ Fixed issues (1):

Rule	Count
invalid-assignment	1

First entries

tests/run_agent/test_credits_notices_toggle.py:76: [invalid-assignment] invalid-assignment: Object of type `None` is not assignable to attribute `_credits_session_start_micros` of type `int`

Unchanged: 6037 pre-existing issues carried over.

Diagnostics are surfaced as warnings — this check never fails the build.