feat: full Discord-style presence tracking

- Announce online immediately on app startup
- Idle detection: unavailable after 10 min of no input, online on return
- Tab visibility: unavailable when hidden, online when focused again
- Page close: offline via fetch+keepalive (survives unload without bfcache penalty)
- hidePresence setting: broadcasts offline and stops all tracking
- Added 'Hide Online Status' toggle in General settings

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -789,6 +789,7 @@ function Editor() {
   const [enterForNewline, setEnterForNewline] = useSetting(settingsAtom, 'enterForNewline');
   const [isMarkdown, setIsMarkdown] = useSetting(settingsAtom, 'isMarkdown');
   const [hideActivity, setHideActivity] = useSetting(settingsAtom, 'hideActivity');
+  const [hidePresence, setHidePresence] = useSetting(settingsAtom, 'hidePresence');
   const [editorToolbar, setEditorToolbar] = useSetting(settingsAtom, 'editorToolbar');
 
   return (
@@ -823,6 +824,13 @@ function Editor() {
           after={<Switch variant="Primary" value={hideActivity} onChange={setHideActivity} />}
         />
       </SequenceCard>
+      <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
+        <SettingTile
+          title="Hide Online Status"
+          description="Appear offline to everyone while still using the app. Disables online, away, and idle indicators."
+          after={<Switch variant="Primary" value={hidePresence} onChange={setHidePresence} />}
+        />
+      </SequenceCard>
     </Box>
   );
 }