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

webapp/src/components/LogTimeline.tsx

@@ -31,6 +31,10 @@ export function computePresetRange(id: TimeRangePresetId): TimeRange | null {
   return { startMs: now - preset.ms, endMs: now }
 }
 
+function formatTimestamp(ms: number): string {
+  return new Date(ms).toLocaleTimeString()
+}
+
 export function LogTimeline({
   buckets,
   range,
@@ -47,11 +51,14 @@ export function LogTimeline({
   const containerRef = useRef<HTMLDivElement>(null)
   const [dragStartX, setDragStartX] = useState<number | null>(null)
   const [dragCurrentX, setDragCurrentX] = useState<number | null>(null)
+  const [hoverX, setHoverX] = useState<number | null>(null)
 
   const maxCount = Math.max(1, ...buckets.map((b) => b.count))
   const domainStart = range?.startMs ?? buckets[0]?.start_ms ?? 0
   const domainEnd = range?.endMs ?? Date.now()
   const domainSpan = Math.max(1, domainEnd - domainStart)
+  const firstBucket = buckets[0] ?? null
+  const lastBucket = buckets[buckets.length - 1] ?? null
 
   function xToMs(x: number) {
     const rect = containerRef.current?.getBoundingClientRect()
@@ -60,12 +67,25 @@ export function LogTimeline({
     return Math.round(domainStart + ratio * domainSpan)
   }
 
+  function bucketAtOrBefore(ms: number): TimelineBucket | null {
+    let result: TimelineBucket | null = null
+    for (const b of buckets) {
+      if (b.start_ms <= ms) result = b
+      else break
+    }
+    return result ?? firstBucket
+  }
+
   function handleMouseDown(e: React.MouseEvent) {
     setDragStartX(e.clientX)
     setDragCurrentX(e.clientX)
   }
   function handleMouseMove(e: React.MouseEvent) {
-    if (dragStartX !== null) setDragCurrentX(e.clientX)
+    if (dragStartX !== null) {
+      setDragCurrentX(e.clientX)
+    } else {
+      setHoverX(e.clientX)
+    }
   }
   function handleMouseUp() {
     if (dragStartX !== null && dragCurrentX !== null && Math.abs(dragCurrentX - dragStartX) > 3) {
@@ -84,6 +104,8 @@ export function LogTimeline({
         width: Math.abs(dragCurrentX! - dragStartX!),
       }
     : null
+  const hoverBucket = !dragging && hoverX !== null ? bucketAtOrBefore(xToMs(hoverX)) : null
+  const containerLeft = containerRef.current?.getBoundingClientRect().left ?? 0
 
   return (
     <div className="border-b border-border p-3">
@@ -128,6 +150,7 @@ export function LogTimeline({
         onMouseLeave={() => {
           setDragStartX(null)
           setDragCurrentX(null)
+          setHoverX(null)
         }}
       >
         <div className="flex h-full w-full items-end gap-px overflow-hidden px-1">
@@ -136,16 +159,41 @@ export function LogTimeline({
               key={b.start_ms}
               className="min-w-[2px] flex-1 rounded-t bg-accent/70"
               style={{ height: `${(b.count / maxCount) * 100}%` }}
-              title={`${new Date(b.start_ms).toLocaleTimeString()} — ${b.count}`}
             />
           ))}
         </div>
+        {firstBucket && (
+          <div className="pointer-events-none absolute left-1 top-1 whitespace-nowrap text-[10px] text-text-subtle">
+            #0 · {formatTimestamp(firstBucket.start_ms)}
+          </div>
+        )}
+        {lastBucket && (
+          <div className="pointer-events-none absolute right-1 top-1 whitespace-nowrap text-[10px] text-text-subtle">
+            #{buckets.length - 1} · {formatTimestamp(lastBucket.start_ms)}
+          </div>
+        )}
         {selRect && (
           <div
             className="pointer-events-none absolute top-0 h-full border border-accent bg-accent/20"
-            style={{ left: selRect.left - (containerRef.current?.getBoundingClientRect().left ?? 0), width: selRect.width }}
+            style={{ left: selRect.left - containerLeft, width: selRect.width }}
           />
         )}
+        {dragging && (
+          <div
+            className="pointer-events-none absolute top-1 z-10 -translate-x-1/2 whitespace-nowrap rounded border border-accent bg-surface-2 px-1.5 py-0.5 text-[11px] text-text-secondary shadow"
+            style={{ left: (selRect?.left ?? 0) - containerLeft + (selRect?.width ?? 0) / 2 }}
+          >
+            {formatTimestamp(xToMs(dragStartX!))} → {formatTimestamp(xToMs(dragCurrentX!))}
+          </div>
+        )}
+        {!dragging && hoverBucket && hoverX !== null && (
+          <div
+            className="pointer-events-none absolute top-1 z-10 -translate-x-1/2 whitespace-nowrap rounded border border-border-soft bg-surface-2 px-1.5 py-0.5 text-[11px] text-text-muted shadow"
+            style={{ left: hoverX - containerLeft }}
+          >
+            {formatTimestamp(hoverBucket.start_ms)} — {hoverBucket.count}
+          </div>
+        )}
       </div>
     </div>
   )