feat: switch from ttyd to real kitty via Xpra HTML5

Replaced the tmux+ttyd web terminal with actual kitty running through
Xpra's HTML5 streaming. Full GPU rendering, native kitty tabs/splits,
persistent sessions, and multi-client support.

systemd/kitty-web.service

@@ -0,0 +1,30 @@
+[Unit]
+Description=Kitty terminal via Xpra HTML5 (kitty.sethpc.xyz)
+After=network.target
+
+[Service]
+Type=simple
+User=rdp
+Group=rdp
+WorkingDirectory=/home/rdp
+Environment=XDG_RUNTIME_DIR=/run/user/1002
+
+ExecStart=/usr/bin/xpra start \
+    --bind-ws=0.0.0.0:7681 \
+    --start="kitty" \
+    --html=on \
+    --no-notifications \
+    --no-pulseaudio \
+    --no-mdns \
+    --no-printing \
+    --sharing=yes \
+    --readonly=no \
+    --no-daemon
+
+ExecStop=/usr/bin/xpra stop :0
+
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=multi-user.target