Moved the Flatpak display troubleshooting content from readme.md into a standalone troubleshooting.md document, as requested.

Replaced the large inline README section with a single link to the new troubleshooting file to keep the README concise.

troubleshooting.md

@@ -0,0 +1,41 @@
+# Troubleshooting
+
+## Flatpak
+
+If `flatpak run net.zoite.Zoitechat` only prints `Gtk-WARNING **: cannot open display`,
+collect extra diagnostics with:
+
+```bash
+flatpak run --devel --command=sh net.zoite.Zoitechat
+```
+
+Then inside that shell:
+
+```bash
+echo "DISPLAY=$DISPLAY WAYLAND_DISPLAY=$WAYLAND_DISPLAY XDG_SESSION_TYPE=$XDG_SESSION_TYPE"
+xdpyinfo >/tmp/xdpyinfo.log 2>&1 || true
+env G_MESSAGES_DEBUG=all zoitechat 2>&1 | tee /tmp/zoitechat-debug.log
+```
+
+To inspect sandbox permissions from the host:
+
+```bash
+flatpak info --show-permissions net.zoite.Zoitechat
+flatpak override --user --show net.zoite.Zoitechat
+```
+
+If needed, try running with direct access to your active display stack:
+
+```bash
+# X11 sessions
+flatpak override --user --socket=x11 net.zoite.Zoitechat
+
+# Wayland sessions
+flatpak override --user --socket=wayland net.zoite.Zoitechat
+```
+
+You can reset overrides after testing:
+
+```bash
+flatpak override --user --reset net.zoite.Zoitechat
+```