Add native self-hosted instance connection to fluxer_desktop

Trimmed monorepo checkout (fluxer_desktop + packages/voice_engine_v2 +
tools/ci) with a "Connect to a Different Server" menu item and popout
that lets the desktop app switch to any self-hosted Fluxer instance,
plus fixes for well-known discovery on single-domain self-hosted
deployments and a false-positive ERR_ABORTED on same-origin client
redirects during the switch. Defaults to chat.fluxr.chat and uses an
isolated userData directory from the official build.

fluxer_desktop/packaging/linux/app.fluxer.Fluxer.desktop

@@ -0,0 +1,26 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+[Desktop Entry]
+Name=Fluxer
+GenericName=Instant Messenger
+Comment=Instant messaging and VoIP
+Exec=app.fluxer.Fluxer %U
+TryExec=app.fluxer.Fluxer
+Icon=app.fluxer.Fluxer
+Type=Application
+Categories=Network;InstantMessaging;Chat;
+Keywords=chat;im;messaging;messenger;voip;voice;video;call;
+StartupWMClass=app.fluxer.Fluxer
+SingleMainWindow=true
+StartupNotify=true
+MimeType=x-scheme-handler/fluxer;
+Terminal=false
+X-GNOME-UsesNotifications=true
+Actions=open-settings;new-dm;
+
+[Desktop Action open-settings]
+Name=Open Settings
+Exec=app.fluxer.Fluxer --fluxer-task=open-settings %U
+
+[Desktop Action new-dm]
+Name=New Direct Message
+Exec=app.fluxer.Fluxer --fluxer-task=new-dm %U