Address touch review: one coarse-pointer signal + stable nudge callbacks

Gadfly on #104:
- The handles keyed off `pointer: coarse` but the NudgePad off `md:hidden`
  (viewport), so a large touchscreen or a narrow mouse window got them
  disagreeing. Extracted one `isCoarsePointer` in shared.ts that both use —
  the pad now shows on a coarse pointer, same as the bigger handles.
- Wrapped commitLater/nudgeSelected in useCallback([]) — stable identity, so
  NudgePad doesn't re-render each parent render, and the mount-once keydown
  effect capturing nudgeSelected is now explicitly safe (a comment spells out
  the refs-only invariant that makes the empty-deps capture correct).
- isCoarsePointer's optional-chained matchMedia keeps it false (mouse
  defaults) under test/SSR, addressing the constants-file testability note.

tsc + build green.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

web/src/editor/shared.ts

@@ -3,12 +3,17 @@
 // one place instead of drifting between files.
 
 export const SELECT_COLOR = '#2f7a3e' // selection stroke/handles
-// On-screen size of a drag/resize handle. Bigger on a coarse pointer (touch) so
-// a fingertip can actually grab a resize corner or the rotate knob — 12px is fine
-// for a mouse but frustrating for a thumb (#104). Read once at load; a device
-// doesn't switch its primary pointer mid-session.
-export const HANDLE_PX =
-  typeof window !== 'undefined' && window.matchMedia?.('(pointer: coarse)').matches ? 22 : 12
+// Whether the primary pointer is a fingertip rather than a mouse — the one signal
+// the touch affordances key off (bigger handles here, the on-screen nudge pad in
+// the editor), so they can't disagree about what "touch" means. Read once at
+// load; a device doesn't switch its primary pointer mid-session, and the optional
+// chain keeps it false (mouse defaults) under test / SSR where matchMedia is absent.
+export const isCoarsePointer =
+  typeof window !== 'undefined' && !!window.matchMedia?.('(pointer: coarse)').matches
+// On-screen size of a drag/resize handle. Bigger on touch so a fingertip can
+// actually grab a resize corner or the rotate knob — 12px is fine for a mouse but
+// frustrating for a thumb (#104).
+export const HANDLE_PX = isCoarsePointer ? 22 : 12
 export const MIN_RADIUS_CM = 1 // smallest plop radius
 export const DIMMED_OPACITY = 0.4 // non-focused objects/plops in focus mode