Add timeline start/end markers, cursor hover, and live drag readout (ticket 031)

LogTimeline now always shows the first/last bucket's index and timestamp,
a hover readout that tracks the cursor with the bucket at/before it, and
a live-updating start/end readout while dragging a selection.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

tickets/031-timeline-markers-and-hover.md

@@ -1,40 +0,0 @@
-# 031 — Timeline start/end markers, cursor hover, and live drag readout
-
-## Summary
-
-`LogTimeline` (`webapp/src/components/LogTimeline.tsx`) shows a bar per
-bucket with only a per-bar `title` tooltip on native hover, and a drag
-selection that's silent until `handleMouseUp` fires `onSelectRange`. Add
-always-visible start/end labels, a hover readout that tracks the cursor, and
-a live-updating readout while dragging.
-
-## Scope
-
-- **Start/end markers**: render the sequence index and timestamp of the
-  first and last visible bucket at the two ends of the timeline (using
-  `domainStart`/`domainEnd`, already computed at
-  `LogTimeline.tsx:52-53`), always visible, not hover-only.
-- **Cursor hover readout**: on `onMouseMove` (already wired for drag at
-  `handleMouseMove`, `LogTimeline.tsx:65`), when not dragging, compute the
-  bucket at or immediately before the cursor via `xToMs` and show its
-  timestamp/count in a readout near the cursor — replacing reliance on the
-  native per-bar `title` attribute (`LogTimeline.tsx:135`).
-- **Live drag readout**: while `dragging` is true (`LogTimeline.tsx:78`), show
-  the resolved start/end (`xToMs(dragStartX)`/`xToMs(dragCurrentX)`) updating
-  on every `handleMouseMove`, not just after `handleMouseUp` resolves the
-  final range — the drag rectangle already updates live (`selRect`,
-  `LogTimeline.tsx:79-84`); this adds the numeric readout next to it.
-
-## Acceptance criteria
-
-- The timeline always shows the sequence/timestamp of its first and last
-  bucket, without needing to hover.
-- Moving the mouse over the timeline (not dragging) shows the timestamp/count
-  of the bucket under (or just before) the cursor, updating as the mouse
-  moves.
-- While dragging, the displayed start/end timestamps update continuously as
-  the drag extends, matching the already-live selection rectangle.
-
-## Depends on
-
-None — builds on the existing `LogTimeline` drag/hover plumbing.