feat: instance-level streaming limits with admin settings panel

Add a server-side instance_settings table (single-row, CHECK(id=1))
that stores admin-configurable streaming bounds: bitrate min/max/step,
allowed resolutions, and allowed framerates.

Backend:
- New instance_settings schema + migrations (is_admin on users, default
  settings row, first-registered-user promoted to admin)
- GET/PATCH /api/settings/streaming endpoints with admin-only writes
  and full input validation including cross-field checks

Frontend:
- settingsStore fetches limits on WebSocket ready, exposes isAdmin flag
- ScreenShareSettingsPopover reads bounds from store instead of
  hardcoded constants, auto-clamps stale localStorage values
- buildScreenShareOptions() clamps bitrate to server limits at build
  time as enforcement backstop
- ServerSettings modal gains a "Streaming" tab (admin-only) with
  bitrate range sliders, resolution/framerate toggles, and save/reset

packages/web/src/components/chat/FriendsPage.test.tsx

@@ -59,6 +59,7 @@ const makeRequest = (overrides: Partial<FriendRequest> = {}): FriendRequest => (
     avatar: null,
     status: 'online',
     customStatus: null,
+    isAdmin: false,
     createdAt: Date.now(),
   },
   ...overrides,
@@ -212,6 +213,7 @@ describe('FriendsPage', () => {
           avatar: null,
           status: 'online',
           customStatus: null,
+          isAdmin: false,
           createdAt: Date.now(),
         },
       });
@@ -256,6 +258,7 @@ describe('FriendsPage', () => {
           avatar: null,
           status: 'online',
           customStatus: null,
+          isAdmin: false,
           createdAt: Date.now(),
         },
       });
@@ -295,6 +298,7 @@ describe('FriendsPage', () => {
           avatar: null,
           status: 'online',
           customStatus: null,
+          isAdmin: false,
           createdAt: Date.now(),
         },
       });