merge: upstream Element Call v0.28.0 into lotus (cinny #201)

Syncs the fork from upstream v0.25.0 to v0.28.0 (196 upstream commits;
v0.27 was skipped upstream), bringing matrix-js-sdk develop up to the
SDK 44 MatrixRTC changes (livekit_alias / oldest_membership removed),
delayed-leave delegation to the SFU, and the HostBridge host interface.

Lotus changes carried across the refactors:
- widget: upstream no longer exports a widget singleton (initializer hands
  initializeWidget()'s result to React). The Lotus host integration
  (src/lotus/*) reads `widget` at call time, so a Lotus-only exported
  reference is set by initializeWidget.
- useTheme: theme classes now go on the root element (the body in the
  widget build); the lotusTheme / lotusTransparent / lotusHostControls
  classes follow it.
- LocalMember: [lotus #46] keep retrying network errors until the delayed
  leave would fire (Math.max), on upstream's restructured timings.
- UrlConfiguration moved to its own file; autoGainControl moved with it.
- errors: upstream switched to i18n.t; SFUTokenRefusedError follows.
- keyboard shortcuts: Lotus host hotkeys still win over focusIsClaimed().
- GroupCallErrorBoundary: the reconnect handler uses the widget reference
  (upstream dropped the widget prop).

Config: v0.26 renamed matrix_rtc_session.delayed_leave_event_* to
delayed_leave.{delay_ms,restart_ms,restart_timeout_ms} with no fallback,
and the embedded build no longer emits a config.json. The production
config (public/element-call/config.json on LXC 106, not in git) must move
to the new keys when this ships, or calls lose the 60 s grace (lotus #46).

Embedded version 0.28.0-lotus.1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

.gitignore

@@ -10,6 +10,9 @@ public/config.json
 backend/synapse_tmp/*
 backend/synapse_tmp_othersite/*
 /coverage
+
+# Transient agent scratch files
+/agent-workspace/
 config.json
 
 # Yarn
@@ -39,3 +42,6 @@ yarn-error.log
 *storybook.log
 storybook-static
 .pnpm-store/
+
+# Nested checkout made by scripts/layered.sh
+/matrix-js-sdk/