Polish: clear-bed, keyboard nudging, empty states, titles, 404 (#18)

- Clear bed: a "Clear (N)" action on a focused plantable object → confirm modal
  (ClearBedModal) → soft-removes every active plop (useClearObject loops PATCHes,
  invalidates once); rows kept with removed_at.
- Keyboard nudging (desktop): arrows move the selected object/plop 1cm, Shift
  10cm; the PATCH is debounced ~400ms on key-idle. Plops nudge in the object's
  local frame and clamp to its bounds; ignored while typing in a field.
- Empty states: hint to place a first bed on an empty field, and to add a plant
  in an empty focused bed (both non-interactive overlays).
- Paper cuts: per-route document titles (usePageTitle on every page), an emoji
  favicon, and a 404 catch-all route (NotFound) inside the app shell.
- Mobile: plop tap targets stay ≥~44px at low zoom via a transparent hit circle.
- Ownership check hoisted to the authoritative ownerId==me so the nudge handler
  can gate on canEdit before the loading early-returns.

Imperial coverage verified across inspectors/forms/cards/picker (all via the
unit-aware helpers); conversion helpers stay in lib/units.ts (their home since #8).

tsc --noEmit clean; 24/24 vitest; production build green.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

web/index.html

@@ -4,6 +4,7 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0, viewport-fit=cover" />
     <meta name="color-scheme" content="light dark" />
+    <link rel="icon" href="data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'><text y='.9em' font-size='90'>%F0%9F%8C%B1</text></svg>" />
     <title>pansy</title>
     <meta name="description" content="Self-hostable garden planner — plan beds, containers, and plops of plants at real scale." />
   </head>