feat(frontend): add brand kit management UI and project binding

Add a brand kit domain model, repository, and dedicated page/route for
managing brand kits, plus a BrandKitSelector used on the home page and
canvas workspace. Home project creation and the workspace can attach a
brand kit to a project, with new i18n strings and a side-nav entry.

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

frontend/src/ui/styles.css

@@ -1291,6 +1291,9 @@ button:disabled {
   height: 48px;
   display: flex;
   align-items: center;
+  gap: 7px;
+  max-width: calc(100% - 220px);
+  min-width: 0;
   pointer-events: auto;
 }
 
@@ -3943,9 +3946,14 @@ button:disabled {
   .workspace-stage-title {
     left: 6px;
     right: 132px;
+    max-width: none;
     min-width: 0;
   }
 
+  .workspace-stage-title .brand-kit-selector-trigger.is-compact {
+    max-width: 104px;
+  }
+
   .canvas-open-account-controls {
     right: 8px;
   }