feat(audio-player): scrub interaction rework — free-flow drag, pixel magnetism, touch pull-away precision (plan §9)

Mouse: drag is free-flow 1:1 (rail only on Shift+drag, now with
window from/to edge labels); clicks land with pixel magnetism
(MAGNET_RADIUS_PX=10) — the old 5-second threshold was sub-pixel on
long books and never fired, which was the 'chapter scrub isn't
accurate' bug. Hover inside the radius locks the tooltip to the line
with a highlight. Shift+click jumps to the nearest line from anywhere.

Touch: drag is an incremental cursor (applyDrag) whose scale tightens
continuously as the finger pulls off the bar (dragScale, exponential
10^(-|dy|/80px)) — the rail is the feedback: window shrinks with the
zoom (touchRailSpan) and lifts toward the finger (railLiftPx).

Dialog retitled 'Shortcuts & interactions' with a Mouse & touch
section driven by POINTER_GUIDE (audio-keys.ts). Old seconds-threshold
snapToChapter deleted. 133 unit tests; 19 Playwright checks on the
KJV (1189ch) and Gardens (34ch) books.

web/src/audio-keys.ts

@@ -213,6 +213,26 @@ export function visibleKeymap(): KeyActionSpec[] {
   return KEYMAP.filter((spec) => !spec.hidden);
 }
 
+// Pointer/wheel/touch interactions (plan §9) are not key events — the
+// dispatcher never sees them — but they belong in the same dialog so
+// every scrub gesture is discoverable in one place. templ x-fors this
+// table through the component's `pointerGuide` property (same binding
+// pattern as speeds/skipChoices).
+export interface PointerGuideRow {
+  // Badge label, e.g. "Shift + drag".
+  label: string;
+  description: string;
+}
+
+export const POINTER_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: "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)" },
+  { label: "Touch + pull away", description: "Scrub, then drag off the bar for fine control" },
+];
+
 // resolveShortcut maps a keydown's (key, altKey) to an action, or null
 // when nothing claims it. Single characters match case-insensitively;
 // Alt-qualified bindings take precedence order for free: a plain key