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/shared/src/types.ts

@@ -7,6 +7,7 @@ export interface User {
   avatar: string | null;
   status: UserStatus;
   customStatus: string | null;
+  isAdmin: boolean;
   createdAt: number;
 }
 
@@ -366,3 +367,15 @@ export interface SendFriendRequest {
 export interface UpdateFriendRequest {
   status: 'accepted' | 'declined';
 }
+
+// ─── Instance Settings Types ────────────────────────────────────────────────
+
+export interface InstanceStreamingLimits {
+  maxBitrateKbps: number;
+  minBitrateKbps: number;
+  bitrateStepKbps: number;
+  allowedResolutions: number[];
+  allowedFramerates: number[];
+  maxResolution: number;
+  maxFramerate: number;
+}