Touch ergonomics: bigger handles + on-screen nudge pad (#104)

Two touch gaps the audit named:

- Resize/rotate handles were a fixed 12px — fine for a mouse, hard for a
  fingertip. HANDLE_PX is now 22px on a coarse pointer (touch), 12px
  otherwise. Read once at load.
- Fine positioning was keyboard-only (arrow-nudge), which a phone can't
  reach and a drag can't do at single-cm precision. Added an on-screen
  NudgePad — a ↑←→↓ d-pad (~40px targets) shown while something's selected
  on a touch layout (md:hidden).

To share behaviour without duplicating the intricate part, extracted
nudgeSelected(dx, dy) from the keyboard handler — the live-geometry update
+ one debounced PATCH (so a burst of nudges from either surface commits
once) + the plop-bounds clamp. The keyboard handler and the pad both call
it. Verified live: the pad moves a selected bed 1cm/tap on mobile, and the
keyboard arrows still nudge on desktop after the refactor.

(The rail-vs-toast layering the issue also lists was resolved by #101 —
the rail is now an in-flow peek, not a fixed sheet the toast could cover.)

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

web/src/editor/shared.ts

@@ -3,7 +3,12 @@
 // one place instead of drifting between files.
 
 export const SELECT_COLOR = '#2f7a3e' // selection stroke/handles
-export const HANDLE_PX = 12 // on-screen size of a drag/resize handle
+// 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
 export const MIN_RADIUS_CM = 1 // smallest plop radius
 export const DIMMED_OPACITY = 0.4 // non-focused objects/plops in focus mode