Add custom module support with persistent library

Introduces a blank/custom module placeholder that users can name and
reuse across patches. Includes:

- custom-module symbol (dashed octagon SVG) in a new Custom category
- IndexedDB-backed custom module library (lib/customModuleLibrary.js)
- Auto-saves named modules to the library on first placement; blocks
  duplicates with an alert
- Saved modules appear in the palette under Custom for quick re-use
  without a name prompt
- Rename from the palette propagates to all matching canvas instances
- Delete from the palette warns if instances exist and offers to remove
  them, cancelling does nothing

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

Author: walmik

app/globals.css

@@ -190,6 +190,62 @@ button {
   box-shadow: var(--card-shadow);
 }
 
+.saved-custom-module {
+  position: relative;
+  cursor: grab;
+}
+
+.custom-module-action {
+  position: absolute;
+  top: 6px;
+  width: 20px;
+  height: 20px;
+  padding: 0;
+  border: none;
+  border-radius: 50%;
+  background: transparent;
+  color: var(--ink);
+  font-size: 14px;
+  line-height: 1;
+  opacity: 0;
+  cursor: pointer;
+  transition: opacity 120ms ease, background 120ms ease;
+}
+
+.custom-module-edit {
+  right: 30px;
+}
+
+.custom-module-delete {
+  right: 6px;
+}
+
+.saved-custom-module:hover .custom-module-action {
+  opacity: 0.5;
+}
+
+.custom-module-edit:hover {
+  opacity: 1 !important;
+  background: rgba(11, 136, 216, 0.12);
+  color: #0b88d8;
+}
+
+.custom-module-delete:hover {
+  opacity: 1 !important;
+  background: rgba(200, 30, 30, 0.12);
+  color: #c01e1e;
+}
+
+html.theme-dark .custom-module-edit:hover {
+  background: rgba(11, 136, 216, 0.2);
+  color: #4daee8;
+}
+
+html.theme-dark .custom-module-delete:hover {
+  background: rgba(220, 50, 50, 0.18);
+  color: #e05555;
+}
+
 .symbol-meta {
   min-width: 0;
 }