fix: pin matrix-js-sdk to a working commit, use our own Element Call fork

The floating github:matrix-org/matrix-js-sdk#develop reference was
resolved to a stale commit missing src/oidc/authorize.ts (breaking the
webpack build). Investigated moving to the current develop HEAD instead
(191908da) - turned out to be worse, not better: that commit renamed the
entire src/oidc/ module to src/oauth/ with a materially different API
(generateOidcAuthorizationUrl/completeAuthorizationCodeGrant/OidcError
don't exist under those names anymore). Rather than porting our OIDC
login code to the new API sight-unseen, pinned to d19cb751 - the last
develop commit before that rename, verified to have both the file at
the expected path and the exact exports apps/web imports.

Also switched the Element Call embedded widget dependency from
upstream's @element-hq/element-call-embedded to our own published fork
(@sorb/threadnet-call-embedded@0.19.2-threadnet.5) - a full rebuild was
otherwise silently bundling the unmodified upstream widget, discarding
every Element Call customization (Video-tab quality settings, German
translations, codec list fix).

apps/web/webpack.config.ts

@@ -725,7 +725,7 @@ export default (env: string, argv: Record<string, any>): webpack.Configuration =
                     // Element Call embedded widget
                     {
                         from: "**",
-                        context: path.join(getPackageRoot("@element-hq/element-call-embedded"), "dist"),
+                        context: path.join(getPackageRoot("@sorb/threadnet-call-embedded"), "dist"),
                         to: path.join(__dirname, "webapp", "widgets", "element-call"),
                     },
                     // Mobile guide assets