refactor(frontend): audit fixes - accessibility, colors, and UX

Accessibility:
- Add focus-visible indicators to all interactive elements (buttons, inputs)
- Create accessible CreateUserModal to replace window.prompt()
- Add aria-labels to icon buttons for screen readers

Colors & Theming:
- Move primary color to CSS variable (--primary)
- Remove hard-coded #3b82f6 from tailwind config
- Clean up color consistency across theme

UX:
- Replace prompt-based user creation with proper modal form
- Implement inline field validation with error messages
- Add loading state during form submission
- Improve visual hierarchy and spacing

Performance:
- Remove bootstrap-icons dependency (duplicate with lucide-react)

Design:
- Reduce backdrop-blur overuse (remove from overlay scrim, StatCard)
- Improve AdminOverlay responsive design

frontend/package.json

@@ -10,7 +10,6 @@
   },
   "dependencies": {
     "axios": "^1.15.0",
-    "bootstrap-icons": "^1.11.3",
     "clsx": "^2.1.1",
     "dexie": "^4.4.2",
     "html5-qrcode": "^2.3.8",