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/tailwind.config.ts

@@ -11,8 +11,8 @@ const config: Config = {
         background: "var(--background)",
         foreground: "var(--foreground)",
         primary: {
-          DEFAULT: "#3b82f6",
-          foreground: "#ffffff",
+          DEFAULT: "var(--primary)",
+          foreground: "var(--primary-foreground)",
         },
       },
       keyframes: {