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

.claude/settings.local.json

@@ -34,7 +34,8 @@
       "Bash(npm run *)",
       "Bash(npm test *)",
       "Bash(python3 *)",
-      "Bash(ls -lh aInventory-PROD-v*.zip)"
+      "Bash(ls -lh aInventory-PROD-v*.zip)",
+      "Bash(npm install *)"
     ]
   }
 }