feat(privacy): strip EXIF/XMP/IPTC from image uploads by default (#109)

Metadata was only dropped as a side effect of opt-in compression, so a
phone photo carried its GPS fix, camera model and timestamp into the room
and the media store. utils/stripImageMetadata.ts now removes it at the
container level, without touching pixels: JPEG drops APP1/APP13/COM
(writing back a minimal EXIF holding only Orientation when it isn't 1,
so sideways-stored photos still display upright), PNG drops eXIf and the
text chunks XMP lives in, WebP drops EXIF/XMP and clears the VP8X flags.
Other types pass through.

Applied before encryption on every composer path (attach, paste, drop,
share target) and to user/room avatar picks; GIF upload is excluded.
Setting → General → Privacy "Remove Photo Metadata Before Sending",
default on. The upload card says "Photo metadata removed".

Unit tests on generated fixtures with a GPS IFD (JPEG orientation 6,
JPEG + comment, PNG with eXIf + XMP, WebP with EXIF); verified end to
end: the bytes stored by Synapse decode fine and carry only Orientation.

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

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

@@ -1442,6 +1442,7 @@ function Privacy() {
     'warnOnUnverifiedDevices',
   );
   const [stripTracking, setStripTracking] = useSetting(settingsAtom, 'stripTrackingParams');
+  const [stripImageMeta, setStripImageMeta] = useSetting(settingsAtom, 'stripImageMetadata');
 
   return (
     <Box direction="Column" gap="100">
@@ -1453,6 +1454,13 @@ function Privacy() {
           after={<Switch variant="Primary" value={stripTracking} onChange={setStripTracking} />}
         />
       </SequenceCard>
+      <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
+        <SettingTile
+          title="Remove Photo Metadata Before Sending"
+          description="Strip location, camera model and timestamp (EXIF/XMP) from JPEG, PNG and WebP images you send or set as an avatar. Pixels are untouched; the rotation is kept. Videos and GIFs are not covered."
+          after={<Switch variant="Primary" value={stripImageMeta} onChange={setStripImageMeta} />}
+        />
+      </SequenceCard>
       <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
         <SettingTile
           title="Hide Typing & Read Receipts"