Dark theme, roles overhaul, modal New Ticket, My Tickets page, and more

- Dark UI across all pages and components (gray-950/900/800 palette)
- New Ticket is now a centered modal (triggered from sidebar), not a separate page
- Add USER role: view and comment only; AGENT and SERVICE can create/edit tickets
- Only admins can set ticket status to CLOSED (enforced server + UI)
- Add My Tickets page (/my-tickets) showing tickets assigned to current user
- Add queue (category) filter to Dashboard
- Audit log entries are clickable to expand detail; comment body shown as markdown
- Resolved date now includes time (HH:mm) in ticket sidebar
- Store comment body in audit log detail for COMMENT_ADDED and COMMENT_DELETED
- Clarify role descriptions in Admin Users modal
- Remove CI/CD section from README; add full API reference documentation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/prisma/schema.prisma

@@ -10,6 +10,7 @@ datasource db {
 enum Role {
   ADMIN
   AGENT
+  USER
   SERVICE
 }