fix: fallback to legacy media URL on 401, fix EC avatar letter centering

downloadMedia: on 401 (SW session race or allow_redirect hop stripping
auth), retry via /_matrix/media/v3/ which is public on this homeserver
(allow_public_access_to_media_repo: true). Fixes images not loading
after sending, and avatar 401s in call prescreen tiles.

CallEmbed: inject flex-centering CSS for EC 0.19.4 participant avatar
container so the initial letter is correctly centered in its circle.
CSS class names are scoped to _avatarContainer_1mrho_40 in EC 0.19.4.

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

src/app/plugins/call/CallEmbed.ts

@@ -321,6 +321,9 @@ export class CallEmbed {
       'html, body { background: none !important; }',
       `:root { color-scheme: ${this.themeKind}; }`,
       '[style*="height: 0"][style*="z-index: 1"][style*="align-self: center"] { display: none !important; }',
+      // EC 0.19.4: avatar uses line-height centering which breaks in some tile sizes;
+      // override with flexbox for reliable centering of the initial letter.
+      '._avatarContainer_1mrho_40 ._avatar_va14e_8 { display: flex !important; align-items: center !important; justify-content: center !important; line-height: 1 !important; }',
     ].join('\n');
   }