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/server/src/db/index.ts

@@ -174,6 +174,18 @@ function createTables(db: Database.Database): void {
       server_id TEXT NOT NULL REFERENCES servers(id) ON DELETE CASCADE,
       PRIMARY KEY (folder_id, server_id)
     );
+
+    CREATE TABLE IF NOT EXISTS instance_settings (
+      id INTEGER PRIMARY KEY DEFAULT 1 CHECK (id = 1),
+      max_bitrate_kbps INTEGER NOT NULL DEFAULT 20000,
+      min_bitrate_kbps INTEGER NOT NULL DEFAULT 500,
+      bitrate_step_kbps INTEGER NOT NULL DEFAULT 500,
+      allowed_resolutions TEXT NOT NULL DEFAULT '540,720,1080',
+      allowed_framerates TEXT NOT NULL DEFAULT '30,45,60',
+      max_resolution INTEGER NOT NULL DEFAULT 1080,
+      max_framerate INTEGER NOT NULL DEFAULT 60,
+      updated_at INTEGER NOT NULL DEFAULT 0
+    );
   `);
 }