Add in-app pixel editor for sprites in admin UI

Build a dependency-free Canvas2D pixel editor integrated into the admin
Sprites tab. Each sprite card gains an EDIT button that opens a full-screen
editor for the sprite at its native 500x333 resolution.

Tools: pencil (1-8px brush, Bresenham interpolation), eraser, color picker
(palette + native input + pick-from-canvas), fill bucket (flood fill), pan.
Features: cursor-centered zoom, space-drag pan, undo/redo (snapshot per
stroke), pixel grid overlay, transparency checkerboard, load existing sprite,
and keyboard shortcuts (B/E/I/F, Space, +/-/0, Ctrl+Z/Y, G, Esc).

Export reuses the existing POST /api/admin/sprites/{type} upload endpoint via
srcCanvas.toBlob, so no server changes are required.

Closes #5

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

admin/admin.js

@@ -522,6 +522,7 @@
                     <div class="sprite-categories">${sprite.categories || ''}</div>
                     <div class="sprite-preview">${imgHtml}</div>
                     <div class="sprite-btn-row">
+                        <button class="sprite-upload-label sprite-edit-btn" data-type="${sprite.type}">EDIT</button>
                         <label class="sprite-upload-label">
                             UPLOAD
                             <input type="file" class="sprite-upload-input" accept=".png,image/png" data-type="${sprite.type}">
@@ -530,6 +531,15 @@
                     </div>
                 `;
 
+                const editBtn = card.querySelector('.sprite-edit-btn');
+                editBtn.addEventListener('click', () => {
+                    if (window.PixelEditor) {
+                        window.PixelEditor.open(sprite.type, { onSave: loadSprites });
+                    } else {
+                        toast('Pixel editor not loaded', true);
+                    }
+                });
+
                 const fileInput = card.querySelector('.sprite-upload-input');
                 fileInput.addEventListener('change', async (e) => {
                     const file = e.target.files[0];