Add Images admin section for managing source image uses

New top-level nav section showing all source images in a list view
with checkboxes to edit the uses array (grid, bin) per image. Allows
correcting wrongly-tagged images without code changes.

Server PUT /api/source-images/:id was already in place; re-added the
frontend API wrapper that was prematurely removed.

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

public/index.html

@@ -17,6 +17,7 @@
 			<button class="nav-btn" data-section="grids">Grids</button>
 			<button class="nav-btn" data-section="templates">Templates</button>
 			<button class="nav-btn" data-section="bins">Bins</button>
+			<button class="nav-btn" data-section="images">Images</button>
 		</nav>
 		<div class="maint-menu" id="maint-menu">
 			<button class="maint-toggle" id="maint-toggle" title="Maintenance">⚙</button>