fix: resolve five accessibility problems

[rahulkr182]

What does this change?

Fixes #99

Addresses the five accessibility issues identified in the issue:

1. Dark mode border contrast: Increased --border, --input, and --sidebar-border from 220 12% 16% to 220 12% 40% in src/index.css. The contrast ratio against --background: 220 15% 6% is now 3.13:1 (and 3.02:1 against --card: 220 15% 8%), hitting the WCAG 2.1 3:1 non-text contrast requirement.
2. Text below 12px: Replaced all instances of text-[10px] and text-[11px] across src/pages/PromptDetail.tsx, src/components/prompts/PromptCard.tsx, and src/components/prompts/SharePromptDialog.tsx with text-xs (12px / 0.75rem).
3. Hardcoded black over user images: Replaced text-black on the mobile menu trigger in src/components/prompts/PromptCard.tsx with an accessible frosted button (rounded-full bg-background/80 hover:bg-background/90 text-foreground backdrop-blur-sm border border-border/50 shadow-sm), guaranteeing legibility over both dark and light user images.
4. Reduced motion: Added a global @media (prefers-reduced-motion: reduce) block in src/index.css setting animation/transition durations to 0.01ms and scroll-behavior to auto.
5. Delete confirmation modal: Replaced the raw hand-rolled fixed inset-0 modal in src/components/prompts/PromptCard.tsx with Radix UI Dialog, DialogContent, DialogHeader, DialogTitle, DialogDescription, and DialogFooter, giving it focus trapping, Escape-to-close, scroll locking, and accessible dialog roles.
6. Accessibility automated tests: Added tests in src/components/prompts/PromptCard.test.tsx and src/accessibility.test.ts to assert contrast ratio >= 3:1, reduced motion styles, absence of sub-12px text, and dialog focus trap semantics.

Why?

• Dark mode borders were virtually invisible at ~1.3:1 contrast against the background.
• Sub-12px text made muted metadata difficult to read for people with low vision.
• Hardcoded black icon without a solid backing became unreadable when overlaid on dark user-uploaded images.
• Animations did not honor the system's prefers-reduced-motion accessibility setting for vestibular disorders.
• The prompt delete action used a raw div modal lacking focus trap, Escape key handling, and ARIA dialog semantics.

How was it tested?

• Added automated tests in src/components/prompts/PromptCard.test.tsx to verify Radix dialog accessibility and mobile menu overlay styling.
• Added automated tests in src/accessibility.test.ts checking WCAG contrast ratio, prefers-reduced-motion declaration, and text sizing.
• Ran all CI validation commands locally:
  • npm run lint (0 errors)
  • npm run typecheck (0 errors)
  • npm test (all 16 test files and 85 tests passed)
  • npm run build (production build succeeded)
  • npm run db:schema:check (up to date)

Checklist

• npm run lint passes
• npm run typecheck passes
• npm test passes
• npm run build passes
• Any new root-relative asset (/foo.png) is in public/, not src/assets/
• No credentials, keys, or .env files are included
• I've read the CLA in CONTRIBUTING.md