feat(privacy): strip tracking parameters from links on paste, send and render (#103)

Shared links routinely carry ad/analytics identifiers (utm_*, fbclid, gclid,
YouTube si=, Amazon ref=/tag=, X s=/t=, TikTok _r/_t, …) that tie every
recipient's click back to the person who shared the link. New
src/app/utils/urlTracking.ts is a pure, local stripper: a global list +
utm_/pk_/matomo_ prefixes, plus host-scoped rules so e.g. `si` is only
removed on youtube/spotify. matrix.to and non-http(s) schemes are never
rewritten; unparseable input is returned unchanged; Amazon's `th`/`psc`
variant selectors are deliberately kept. 13 unit tests.

Wired at three points, all behind a new Settings → Privacy toggle
(`stripTrackingParams`, default on):
- paste: plain-text pastes are cleaned and re-inserted through Slate's own
  insertData so multi-line pastes still split into paragraphs;
- send: RoomInput submit + schedule paths and MessageEditor saves clean both
  `body` and `formatted_body` (the HTML variant unescapes `&` around each
  URL and re-escapes it so the markup is untouched);
- render: linkify `formatHref`/`format` and explicit `<a href>` in
  formatted_body are cleaned, so links sent from other clients are safe to
  click too. LINKIFY_OPTS is spread into memoised per-timeline objects, so
  the toggle is a module flag kept current by ClientNonUIFeatures.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/settings/general/General.tsx

@@ -1439,10 +1439,18 @@ function Privacy() {
     settingsAtom,
     'warnOnUnverifiedDevices',
   );
+  const [stripTracking, setStripTracking] = useSetting(settingsAtom, 'stripTrackingParams');
 
   return (
     <Box direction="Column" gap="100">
       <Text size="L400">Privacy</Text>
+      <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
+        <SettingTile
+          title="Strip Tracking Parameters from Links"
+          description="Remove utm_, fbclid, YouTube si= and other ad/analytics identifiers from links you paste or send, and from links shown in chat. Runs entirely on this device."
+          after={<Switch variant="Primary" value={stripTracking} onChange={setStripTracking} />}
+        />
+      </SequenceCard>
       <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
         <SettingTile
           title="Hide Typing & Read Receipts"