Update Forgot Password form to new design system

**Description:**
Update `forgot-password-form.tsx` to replace all blue color classes with the new gray design system. Keep form submission and validation logic unchanged.

**Acceptance Criteria:**
- [ ] Blue classes replaced with gray equivalents
- [ ] Buttons use `bg-gray-900` / `hover:bg-gray-800`
- [ ] No functional changes

**File(s):** `frontend/components/auth/forgot-password-form.tsx`