feat: ship 100 Microban levels with overhauled UI and modular core

Replace the 3 hand-crafted JSON levels with the full Microban set (100 solvable
puzzles by David W. Skinner) stored as XSB text and parsed at runtime. Rework
the game into kebab-case modules under a 200-LOC budget:

- core/level-parser: XSB parse with flood-fill interior detection
- core/board-model: pure move/undo/win logic, Phaser-free
- core/progress-store: localStorage persistence with graceful fallback
- core/theme: Nord palette, fonts, responsive tile sizer
- ui/button-factory: one rounded button impl with hover/press
- ui/board-renderer: animated tile/wall/goal/box/player drawing
- scenes: menu, paginated level select (5x4 grid, 5 pages), game

Add WASD movement, U/Z undo, R restart, Esc to menu, live move counter,
best-move tracking per level, win overlay with next/levels actions, and a
radial-gradient CSS backdrop.

Drop the dead Arcade Physics wiring, the broken manual shutdown/destroy code,
the unused main.js self-import, and the hardcoded 3-level registry state.

Add docs/ and refresh README.

docs/codebase-summary.md

@@ -0,0 +1,43 @@
+# Codebase Summary
+
+## Layout
+```
+src/
+├── main.js                         # Entry: boots Phaser on #game-container
+└── game/
+    ├── main.js                     # Phaser Game config + scene registration
+    ├── data/
+    │   └── microban-levels.js      # 100 XSB level strings
+    ├── core/
+    │   ├── level-parser.js         # XSB text → {walls, targets, boxes, player, floors}
+    │   ├── board-model.js          # Pure game state + move/undo/win logic
+    │   ├── progress-store.js       # localStorage persistence (completed + best moves)
+    │   └── theme.js                # Nord color palette, fonts, tile-size helper
+    ├── ui/
+    │   ├── button-factory.js       # Reusable rounded button with hover/press
+    │   └── board-renderer.js       # Draws the board + animates moves
+    └── scenes/
+        ├── menu-scene.js           # Title, play, progress counter, keybinds
+        ├── level-scene.js          # Paginated 5×4 level grid (5 pages × 20)
+        └── game-scene.js           # Gameplay: input, HUD, win overlay
+
+public/
+├── style.css                       # Page background + container shadow
+├── favicon.png
+└── assets/                         # bg.png, logo.png (reserved for future use)
+```
+
+## Data flow
+1. `main.js` creates the Phaser Game with Menu / Level / Game scenes.
+2. `level-scene` writes `registry.currentLevel` on click, starts `GameScene`.
+3. `game-scene` reads the level index, calls `parseLevel()` on the XSB string, builds a `BoardModel`, hands it to `BoardRenderer`.
+4. Input → `BoardModel.tryMove()` → `BoardRenderer.animateMove()` → win check → `progressStore.recordCompletion()`.
+
+## Key design choices
+- **XSB at runtime**: levels are stored as the standard Sokoban text format; the parser flood-fills interior floor tiles so the renderer only paints inside the puzzle shape.
+- **Pure `BoardModel`**: move/undo/win logic has zero Phaser dependencies — trivial to unit-test later.
+- **No Arcade physics**: the original version wired colliders but moved sprites by tween; the physics system did nothing. Removed.
+- **One button factory**: every scene goes through `createButton()` so hover/press feels identical everywhere.
+
+## File size budget
+Every file under 200 LOC per development rules. Data file (`microban-levels.js`) exceeds that because it's pure data, not logic.