feat(calls): configurable ringtone volume in Settings (Bug #4 partial)

Adds 'Ringtone Volume' slider (0–100, default 70%) to Settings → Calls.
The IncomingCall audio element reads the setting and applies it as
audioElement.volume before playing, replacing the implicit browser
default of 1.0.

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

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

@@ -1234,6 +1234,7 @@ function Calls() {
     settingsAtom,
     'callJoinLeaveSound',
   );
+  const [ringtoneVolume, setRingtoneVolume] = useSetting(settingsAtom, 'ringtoneVolume');
 
   const handleJoinLeaveSoundChange = (value: 'off' | 'chime' | 'soft' | 'retro') => {
     setCallJoinLeaveSound(value);
@@ -1565,6 +1566,29 @@ function Calls() {
           />
         )}
       </SequenceCard>
+      <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
+        <SettingTile
+          title="Ringtone Volume"
+          description="Volume of the incoming call ringtone."
+          after={
+            <Box direction="Row" alignItems="Center" gap="200" style={{ minWidth: '160px' }}>
+              <input
+                type="range"
+                min="0"
+                max="100"
+                step="5"
+                value={ringtoneVolume}
+                onChange={(e) => setRingtoneVolume(parseInt(e.target.value, 10))}
+                aria-label="Ringtone volume"
+                style={{ flex: 1, accentColor: 'var(--accent-orange)' }}
+              />
+              <Text size="T200" style={{ minWidth: '32px', textAlign: 'right' }}>
+                {ringtoneVolume}%
+              </Text>
+            </Box>
+          }
+        />
+      </SequenceCard>
       <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
         <SettingTile
           title="Join & Leave Sounds"