feat: add multiple admin support for rooms

[buildingvibes]

Summary

Resolves #27 — Multiple Admin Support

This PR introduces a full multiple admin system for Crowdify rooms:

• OWNER role: Room creators are now assigned an OWNER role (distinct from ADMIN), ensuring they can never be demoted and retain full control
• Admin check via RoomUser: The isAdmin check in /api/room/[roomId] now queries RoomUser.role instead of comparing against room.adminId, enabling multiple admins per room
• Promote/Demote API (/api/room/[roomId]/admins): Room owners can promote any room member to ADMIN or demote them back to USER. Owners cannot be modified.
• Room join tracking (/api/room/[roomId]/join): When users enter a room, a RoomUser record is created (upsert), so all participants are tracked for role management
• Admin Panel UI: A new AdminPanel modal component (accessible via "Manage" button in the top bar) shows all room members with their roles and allows the owner to promote/demote users with a single click
• Prisma migration: Adds the OWNER value to the Role enum and migrates existing room creators from ADMIN to OWNER

Changes

File	Change
prisma/schema.prisma	Added OWNER to Role enum
prisma/migrations/20250212000000_multiple_admin_support/	Migration for OWNER enum + data migration
app/api/room/create/route.ts	Room creator gets OWNER role instead of ADMIN
app/api/room/[roomId]/route.ts	Admin check uses RoomUser.role, returns isOwner flag
app/api/room/[roomId]/admins/route.ts	New — GET lists members, POST promotes/demotes
app/api/room/[roomId]/join/route.ts	New — POST creates RoomUser on room join
app/context/WebContext.tsx	Exposes isOwner, roomUsers, promoteToAdmin, demoteFromAdmin; auto-joins room
app/components/AdminPanel.tsx	New — Modal UI for managing room member roles
app/components/Topbar.tsx	Added "Manage" button for owners to open AdminPanel

Test plan

• Create a new room — verify the creator is assigned OWNER role
• Join the room with a second account — verify a RoomUser with USER role is created
• As owner, open the admin panel and promote the second user to ADMIN
• Verify the promoted user now sees admin controls (skip song, chat pause toggle, song add toggle)
• Demote the user back to USER and verify admin controls are removed
• Verify the owner cannot be demoted or modified
• Verify non-owner admins cannot promote/demote other users

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