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/app/globals.css

@@ -2,12 +2,12 @@
 @tailwind components;
 @tailwind utilities;
 
-@import "bootstrap-icons/font/bootstrap-icons.css";
-
 :root {
   /* slate-950 forced as default to prevent white flash */
   --background: #020617;
   --foreground: #f1f5f9;
+  --primary: #3b82f6;
+  --primary-foreground: #ffffff;
 }
 
 body {