Wire up fields and contents UI for bins and bin types

- build_field_editor() helper: reusable field row editor shared by
  bin editor, bin type dialog (open_component_dialog still uses its own)
- open_bin_editor: tabs (Corners|Fields|Contents), field editor on
  Fields tab, content list on Contents tab, save always persists fields
- open_bin_type_dialog: field editor appended below existing form fields
- render_bin_contents / open_bin_content_dialog: content item CRUD
  (component ref or free-text name, quantity, notes); add/edit/delete
  update all_bins immediately without closing the editor
- bc-cancel / bc-save handlers registered in init()
- bin-content-row CSS

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

public/style.css

@@ -2033,3 +2033,36 @@ nav {
 	text-overflow: ellipsis;
 	white-space: nowrap;
 }
+
+.bin-content-row {
+	display: flex;
+	align-items: center;
+	gap: 0.75rem;
+	padding: 0.4rem 0;
+	border-bottom: 1px solid var(--border);
+}
+
+.bin-content-name {
+	flex: 1;
+	font-weight: 500;
+	min-width: 0;
+	overflow: hidden;
+	text-overflow: ellipsis;
+	white-space: nowrap;
+}
+
+.bin-content-qty {
+	font-size: 0.85rem;
+	color: var(--text-muted);
+	flex-shrink: 0;
+}
+
+.bin-content-notes {
+	font-size: 0.8rem;
+	color: var(--text-faint);
+	flex: 1;
+	min-width: 0;
+	overflow: hidden;
+	text-overflow: ellipsis;
+	white-space: nowrap;
+}