Add resizable room panel, searchable user picker, and direct room membership

Room info panel is now user-resizable (fixing a layout clip at narrow
widths), and every user-selection spot (room membership, admin ownership
transfer) uses a new searchable UserPicker instead of raw text input or
prompt(). Member rows fold role + actions into a single inline dropdown
instead of a row of buttons, so the member list stays usable as rooms grow.

Room invites (the accept/decline flow) are replaced by adding a user to a
room directly -- an admin/owner picks someone and they're a member
immediately, with a "you've been added" notification email instead of an
invite email. Drops the now-unused room_invites table.

backend/app/routers/users.py

@@ -2,16 +2,31 @@ import uuid
 
 from fastapi import APIRouter, Depends, HTTPException
 from fastapi.responses import FileResponse
+from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.database import get_db
 from app.dependencies import get_current_user
 from app.models import User
+from app.schemas.user import UserDirectoryRead
 from app.storage import UPLOADS_DIR
 
 router = APIRouter(prefix="/api/users", tags=["users"])
 
 
+@router.get("", response_model=list[UserDirectoryRead])
+async def list_users_directory_endpoint(
+    current_user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    result = await db.execute(
+        select(User)
+        .where(User.is_active.is_(True), User.is_bot.is_(False))
+        .order_by(User.username)
+    )
+    return list(result.scalars().all())
+
+
 @router.get("/{user_id}/avatar")
 async def get_user_avatar_endpoint(
     user_id: uuid.UUID,