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-scrubber.ts

@@ -79,7 +79,9 @@ export function snapToChapterPx(
 // thumb, in bar pixels, for it to count as grabbing the HANDLE rather
 // than clicking the track. A grab never teleports the position — not
 // to the click X (free-drag behavior) and not to a chapter line
-// (Shift+click behavior) — it drags RELATIVE to where the handle is.
+// (Shift+click behavior): a plain grab drags RELATIVE to where the
+// handle is, and Shift+grab opens the precision rail anchored at the
+// current position.
 export const GRAB_SLOP_PX = 12;
 
 // isHandleGrab reports whether a pointerdown at clientX lands on the
@@ -97,24 +99,19 @@ export function isHandleGrab(
   return Math.abs(clientX - thumbX) <= slopPx;
 }
 
-// GRAB_FINE_SCALE is the Shift+grab travel factor: the same relative
-// drag crawls at quarter speed for precise placement.
-export const GRAB_FINE_SCALE = 0.25;
-
 // applyGrab advances a HANDLE drag by one pointermove: incremental
-// (like applyDrag, a scale change never teleports the cursor), but
-// with a FIXED scale instead of a pull-away zoom — the mouse is always
-// on the bar, so Shift is the precision control, not vertical offset.
+// (like applyDrag, so the cursor never teleports), 1:1 with pointer
+// travel. (Shift+grab uses the precision rail instead of a scaled
+// drag — see scrubPointerDown.)
 export function applyGrab(
   position: number,
   dxPx: number,
   pxPerSecond: number,
   duration: number,
-  scale = 1,
 ): number {
   const clamped = Math.min(duration, Math.max(0, position));
   if (pxPerSecond <= 0 || duration <= 0) return clamped;
-  return Math.min(duration, Math.max(0, clamped + (dxPx / pxPerSecond) * scale));
+  return Math.min(duration, Math.max(0, clamped + dxPx / pxPerSecond));
 }
 
 // ---- §9 touch precision: vertical pull-away = progressive zoom ----