fix(ui): stop settings save echoes from clobbering theme preferences

Every updateDesktopSettings PUT replays the server's full settings
document as an openchamber:settings-synced event, and the theme listener
adopted it unconditionally - so switching sessions across directories
(any lastDirectory/activeProjectId write) could flip the theme to
whatever the server document held at that moment. A bootstrap GET with
missing theme fields made it worse: materializeAuthoritativeUiSettings
invented useSystemTheme + openchamber defaults and the persist effect
wrote them back to the server and the scoped localStorage entry.

Theme is now adopted only from bootstrap-grade syncs (the renamed
bootstrap flag, formerly adoptWorkspace), missing fields mean 'not set'
and keep the current preference, and the materializer no longer invents
theme defaults. Cross-window same-instance theme sync still rides the
scoped-key storage event; runtime endpoint switches still adopt the new
instance's server theme.

Closes the 'theme flashes to OpenChamber when switching sessions' report;
same family as the pre-#2897 'color mode forced to light' symptom.

packages/ui/src/components/auth/SessionAuthGate.tsx

@@ -602,7 +602,7 @@ export const SessionAuthGate: React.FC<SessionAuthGateProps> = ({
       hasBootstrapResyncedRef.current = true;
       void (async () => {
         await initializeAppearancePreferences();
-        await syncDesktopSettings({ adoptWorkspace: isBootstrapResync });
+        await syncDesktopSettings({ bootstrap: isBootstrapResync });
         if (isBootstrapResync) {
           await applyPersistedDirectoryPreferences();
         }