fix(audio): grabbing the playhead thumb no longer teleports it

A pointerdown on the scrub bar unconditionally mapped the click X to a
position, so grabbing the handle itself made the playhead jump — worst
with Shift, whose branch force-snapped to the nearest chapter line
before the drag even started. The user's complaint: the handle click
flowed through to the track-click behavior behind it.

The bar now hit-tests the thumb first (GRAB_SLOP_PX, a pixel test like
the chapter magnetism — a seconds threshold goes sub-pixel on long
books). A grab enters a new "grab" drag mode: the position stays where
it is until the pointer moves, then advances RELATIVELY via the new
applyGrab helper (incremental like the touch cursor, fixed scale — the
mouse is always on the bar, so Shift = quarter-speed fine crawl instead
of a pull-away zoom).

Track clicks keep today's behavior exactly: click = jump with chapter
magnetism, Shift+click = nearest chapter line, Shift+drag = precision
rail, touch = incremental pull-away. MOUSE_GUIDE gains the two grab
rows so the dialog documents the gesture; new pure helpers are unit-
tested alongside the existing scrub math.

web/src/audio-keys.ts

@@ -229,6 +229,11 @@ export interface PointerGuideRow {
 export const MOUSE_GUIDE: readonly PointerGuideRow[] = [
   { label: "Click", description: "Seek — lands on a chapter line within a pixel's reach" },
   { label: "Drag", description: "Free scrub across the whole bar" },
+  {
+    label: "Grab handle",
+    description: "Drag the playhead from where it is — nothing moves until you do",
+  },
+  { label: "Shift + grab", description: "Same handle drag at quarter speed for fine placement" },
   { label: "Shift + click", description: "Jump to the nearest chapter line" },
   { label: "Shift + drag", description: "Precision rail (±5 min) around the nearest line" },
   { label: "Wheel", description: "Nudge ±5s (Shift: ±60s)" },