fix: glassmorphism sidebar background visibility + image upload cleanup

Glassmorphism: the sidebar is a flex sibling of the room view, so
backdrop-filter had nothing behind it to blur. Fix: apply the active
chat background to document.body when glassmorphismSidebar is on
(cleaned up when it's turned off or the component unmounts). Now the
sidebar blurs through the same background pattern as the room view,
making the frosted-glass effect obvious.

Image upload cleanup: delete the pre-uploaded original MXC from the
homeserver after the compressed version is successfully uploaded
(Synapse 1.97+ DELETE /_matrix/client/v1/media/{server}/{mediaId}).
Also delete on cancel when a successful upload is removed by the user.
Both are best-effort — failures are swallowed so UX is unaffected.
Added tryDeleteMxcContent() utility to src/app/utils/matrix.ts.

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

src/app/features/room/RoomInput.tsx

@@ -65,6 +65,7 @@ import {
   getImageInfo,
   getMxIdLocalPart,
   mxcUrlToHttp,
+  tryDeleteMxcContent,
 } from '../../utils/matrix';
 import { compressImage } from '../../utils/imageCompression';
 import { useTypingStatusUpdater } from '../../hooks/useTypingStatusUpdater';
@@ -434,6 +435,8 @@ export const RoomInput = forwardRef<HTMLDivElement, RoomInputProps>(
             });
             const compressedMxc = (uploadRes as { content_uri: string }).content_uri;
             if (compressedMxc) {
+              // Delete the pre-uploaded original so only one copy lives on the server.
+              tryDeleteMxcContent(mx, upload.mxc);
               mxc = compressedMxc;
               // Build a synthetic fileItem that refers to the compressed file so
               // getImageMsgContent picks up the correct dimensions and type.