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/pages/client/share/Share.tsx

@@ -15,6 +15,8 @@ import {
 } from 'folds';
 import { Room } from 'matrix-js-sdk';
 import { useAtomValue, useStore } from 'jotai';
+import { useSetting } from '../../../state/hooks/settings';
+import { settingsAtom } from '../../../state/settings';
 import { Page, PageContent, PageHeader } from '../../../components/page';
 import { useMatrixClient } from '../../../hooks/useMatrixClient';
 import { useMediaAuthentication } from '../../../hooks/useMediaAuthentication';
@@ -53,6 +55,7 @@ export function Share() {
   const [shared, setShared] = useState<Shared | null | undefined>(undefined);
   const [query, setQuery] = useState('');
   const [busy, setBusy] = useState(false);
+  const [stripImageMetadata] = useSetting(settingsAtom, 'stripImageMetadata');
 
   useEffect(() => {
     readSharedPayload()
@@ -80,7 +83,7 @@ export function Share() {
         if (shared.files.length) {
           store.set(roomIdToUploadItemsAtomFamily(room.roomId), {
             type: 'PUT',
-            item: await filesToUploadItems(room, shared.files),
+            item: await filesToUploadItems(room, shared.files, stripImageMetadata),
           });
         }
         const text = shareDraftText(shared.payload);
@@ -95,7 +98,7 @@ export function Share() {
         setBusy(false);
       }
     },
-    [shared, busy, navigateRoom, store],
+    [shared, busy, navigateRoom, store, stripImageMetadata],
   );
 
   return (