fix: repair invite links, social features, messaging + Discord UI overhaul

Phase 1 - Feature Repair:
- Fix member kick/leave: add missing db.delete() call in servers.ts
- Stabilize invite codes: return existing code instead of regenerating
- Fix user search: use LIKE instead of exact match in social.ts
- Wire DM button on FriendsPage to create/navigate to DM channels
- Add cancel outgoing friend request (DELETE endpoint + frontend)
- Add accept/decline friend request actions with WS real-time events
- Fix replyToId persistence in message creation
- Hydrate reactions and replyTo in message queries
- Add joinByCode to API client and serverStore
- Add friend_request_received/accepted WebSocket events

Phase 2 - Discord UI Overhaul:
- Remove stray borders between layout columns
- Replace shadow-sm with shadow-header on content headers
- Replace all bg-gray-*/text-gray-* with Discord color tokens
- Ensure flat color contrast (#1E1F22, #2B2D31, #313338)

Testing:
- Set up vitest + @testing-library/react + jsdom
- Add 17 tests across InviteModal, JoinServer, FriendsPage (all passing)
- Fix vite resolve.extensions to prefer .tsx over stale .js files

packages/web/src/components/ui/Tooltip.tsx

@@ -38,7 +38,7 @@ export function Tooltip({ content, children, position = 'right', delay = 200 }:
       {children}
       {isVisible && (
         <div
-          className={`absolute z-50 px-3 py-1.5 text-sm font-medium text-white bg-gray-900 rounded-md shadow-lg whitespace-nowrap pointer-events-none ${positionClasses[position]}`}
+          className={`absolute z-50 px-3 py-1.5 text-sm font-medium text-white bg-discord-bg-floating rounded-md shadow-elevation-high whitespace-nowrap pointer-events-none ${positionClasses[position]}`}
         >
           {content}
         </div>