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/Slider.tsx

@@ -31,6 +31,11 @@ interface Props {
   max: number;
   step: number;
   disabled?: boolean;
+  /**
+   * Custom formatter for the tooltip label. If not provided, the value is
+   * displayed as a percentage.
+   */
+  tooltipFormatter?: (value: number) => string;
 }
 
 /**
@@ -46,6 +51,7 @@ export const Slider: FC<Props> = ({
   max,
   step,
   disabled,
+  tooltipFormatter,
 }) => {
   const onValueChange = useCallback(
     ([v]: number[]) => onValueChangeProp(v),
@@ -71,7 +77,7 @@ export const Slider: FC<Props> = ({
         <Range className={styles.highlight} />
       </Track>
       {/* Note: This is expected not to be visible on mobile.*/}
-      <Tooltip placement="top" label={Math.round(value * 100).toString() + "%"}>
+      <Tooltip placement="top" label={tooltipFormatter ? tooltipFormatter(value) : Math.round(value * 100).toString() + "%"}>
         <Thumb className={styles.handle} aria-label={label} />
       </Tooltip>
     </Root>