Add zoom/cell-size slider to grid viewer

Images and labels scale together with a range slider (40–300px).
Grid scrolls horizontally when cells exceed viewport width.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/templates.html

@@ -355,12 +355,18 @@
 				<div class="viewer-meta"></div>
 			</div>
 			<div class="grid-viewer-actions">
+				<label class="gv-zoom-label">
+					<span>Size</span>
+					<input type="range" id="gv-zoom" min="40" max="300" value="120" step="10">
+				</label>
 				<button class="btn btn-secondary" id="gv-back">← Back</button>
 				<button class="btn btn-secondary" id="gv-edit-panels">Edit panels</button>
 				<button class="btn btn-danger" id="gv-delete">Delete</button>
 			</div>
 		</div>
-		<div class="grid-cells" id="grid-cells"></div>
+		<div class="grid-cells-wrap">
+			<div class="grid-cells" id="grid-cells"></div>
+		</div>
 	</div>
 </template>