fix(audio): Shift+grab opens the precision rail anchored at the playhead

The first grab implementation degraded Shift+grab to a quarter-speed
relative crawl — losing the ±5-minute rail the user expects when they
grab the handle with Shift held. The rail is back: Shift+grab anchors
railWindow at the CURRENT position (not the nearest chapter line, and
never teleports the playhead on pointerdown); plain grab stays relative
1:1. GRAB_FINE_SCALE is deleted as dead code, and the dialog guide
documents the restored gesture.

web/src/audio-keys.ts

@@ -233,7 +233,10 @@ export const MOUSE_GUIDE: readonly PointerGuideRow[] = [
     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 + grab",
+    description: "Precision rail (±5 min) around the playhead's position",
+  },
   { 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)" },