Add canvas foundation: SVG viewport, pan/zoom/pinch, geometry lib (#9)

The editor's technically-riskiest slice, built against mock data so #11
only adds interactions on top.

- lib/geometry.ts (+ vitest): world<->screen and object-local<->world
  transforms, clampScale, zoomViewportAt (the wheel/pinch "keep the point
  under the cursor stationary" math), zoomToFitRect. 11 geometry tests
  (round-trips, rotation, cursor-anchored zoom, fit) + 6 units tests.
- editor/store.ts: Zustand store for ephemeral editor state — viewport,
  selection, focusedObjectId (selection interactions grow in #11).
- editor/useViewport.ts: @use-gesture wiring — wheel + pinch zoom toward
  the pointer, drag-pan, scale clamped to [0.05, 20] px/cm, and an animated
  fitToRect (eased rAF tween). touch-action:none so the browser doesn't
  fight gestures.
- editor/ObjectShape.tsx: rect/circle centered + rotated, kind default
  colors, name label; non-scaling strokes so outlines stay 1px at any zoom.
- editor/GardenCanvas.tsx: full-size svg, single world <g>, fading 1m grid,
  garden boundary, z-ordered objects; ResizeObserver-driven auto-fit, a
  zoom readout, and a Fit control. Deps: zustand, @use-gesture/react,
  vitest (+ test scripts).
- GardenEditorPage renders the canvas with a mock scene (#11 swaps in
  /full).

Verified in a real browser: wheel zoom keeps the world point under the
cursor stationary (sub-cm drift on a 12m garden), a real drag pans, and
Fit animates to frame the garden. tsc + vitest green.

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

web/package.json

@@ -11,16 +11,20 @@
     "dev": "vite",
     "build": "tsc --noEmit && vite build",
     "typecheck": "tsc --noEmit",
+    "test": "vitest run",
+    "test:watch": "vitest",
     "preview": "vite preview"
   },
   "dependencies": {
     "@tanstack/react-query": "^5.62.0",
     "@tanstack/react-router": "^1.95.0",
+    "@use-gesture/react": "^10.3.1",
     "clsx": "^2.1.1",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
     "tailwind-merge": "^2.6.0",
-    "zod": "^3.24.1"
+    "zod": "^3.24.1",
+    "zustand": "^5.0.14"
   },
   "devDependencies": {
     "@tailwindcss/vite": "^4.0.0",
@@ -30,6 +34,7 @@
     "@vitejs/plugin-react": "^4.3.4",
     "tailwindcss": "^4.0.0",
     "typescript": "^5.7.2",
-    "vite": "^6.0.7"
+    "vite": "^6.0.7",
+    "vitest": "^4.1.10"
   }
 }