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/room/RoomInput.tsx

@@ -279,6 +279,7 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
     const [gifPickerEnabled] = useSetting(settingsAtom, 'gifPickerEnabled');
     // [Gitea #103] Privacy: drop tracking params from links on paste and on send.
     const [stripTracking] = useSetting(settingsAtom, 'stripTrackingParams');
+    const [stripImageMetadata] = useSetting(settingsAtom, 'stripImageMetadata');
     const showGif = (composerToolbarButtons?.showGif ?? true) && gifPickerEnabled;
     const showLocation = composerToolbarButtons?.showLocation ?? true;
     const showPoll = composerToolbarButtons?.showPoll ?? true;
@@ -377,10 +378,10 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
         setUploadBoard(true);
         setSelectedFiles({
           type: 'PUT',
-          item: await filesToUploadItems(room, files),
+          item: await filesToUploadItems(room, files, stripImageMetadata),
         });
       },
-      [setSelectedFiles, room],
+      [setSelectedFiles, room, stripImageMetadata],
     );
     const pickFile = useFilePicker(handleFiles, true);
     const handleFilePaste = useFilePasteHandler(handleFiles);