feat: add user-configurable screen share quality settings UI

Adds a "Screen sharing" section to Settings > Video with controls for:
- Resolution (576p to 4K)
- Framerate (5-60 fps slider)
- Bitrate (0.5-15 Mbps slider)
- Codec (VP8/VP9/H.264/AV1)

Gated behind an "Advanced screen share settings" toggle. When enabled,
settings are passed to LiveKit's setScreenShareEnabled as both capture
constraints and publish options. When disabled, falls back to
config.json media_quality defaults.

Settings are persisted in localStorage via the existing Setting<T>
system. The Slider component is extended with a tooltipFormatter prop
for custom tooltip display.

Inspired by pirosuki's advanced-screen-share-settings branch, but
reimplemented cleanly: settings are read directly in LocalMember.ts
(no signature changes), the existing Slider is extended (no component
duplication), and proper form components are used throughout.

Signed-off-by: Ryan Emmick <ryanemmick4@gmail.com>

src/settings/settings.ts

@@ -150,3 +150,30 @@ export const customLivekitUrl = new Setting<string | null>(
   "custom-livekit-url",
   null,
 );
+
+export type VideoCodec = "vp8" | "vp9" | "h264" | "av1";
+
+export const advancedScreenShare = new Setting<boolean>(
+  "advanced-screen-share",
+  false,
+);
+
+export const screenShareResolution = new Setting<string>(
+  "screen-share-resolution",
+  "1920x1080",
+);
+
+export const screenShareFramerate = new Setting<number>(
+  "screen-share-framerate",
+  30,
+);
+
+export const screenShareBitrate = new Setting<number>(
+  "screen-share-bitrate",
+  5_000_000,
+);
+
+export const screenShareCodec = new Setting<VideoCodec>(
+  "screen-share-codec",
+  "vp9",
+);