style: CSS polish, light mode verification, build fixes

- Add --primary-rgb, --transition-fast, --transition-normal CSS tokens to :root
- Add --primary-rgb override in [data-theme="light"] section
- Fix TS error: remove unused devUser prop from Route elements in App.tsx (API patching via window._devUser makes props redundant)
- Fix TS error: remove unused 'api' import from DevPanel.tsx

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

frontend/src/index.css

@@ -27,6 +27,9 @@
   --shadow-card:    0 10px 25px rgba(0,0,0,0.25);
   --card-shine:     rgba(255,255,255,0.04);
   --scrollbar-bg:   var(--surface-high);
+  --primary-rgb:    75, 183, 248;
+  --transition-fast:   0.15s ease;
+  --transition-normal: 0.3s ease;
 }
 
 /* --- Light Mode --- */
@@ -50,6 +53,7 @@
   --shadow-card:    0 4px 16px rgba(0,0,0,0.10);
   --card-shine:     rgba(255,255,255,0.7);
   --scrollbar-bg:   var(--surface-high);
+  --primary-rgb:    26, 143, 227;
 }
 
 *, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }