Add remote fan tunnel support and rename Waybar widget

Add systemd service to bridge the Pi Unix socket over SSH, make fan.py
fall back to the TCP tunnel, and rename the module from `custom/fan` to
`custom/cooling_pad`. Update README install steps and connection details.

waybar/fan.py

@@ -9,20 +9,43 @@ Usage:
   fan.py auto          switch back to auto mode (on-right-click)
   fan.py duty 75       set manual duty, 0-100
   fan.py setpoint 55   set auto-mode setpoint, 35-85
+
+Connection: the daemon's command socket lives on the Pi. This script uses the
+local Unix socket if present, otherwise a TCP tunnel to it (default
+127.0.0.1:10250, provided by the fan-tunnel systemd service:
+  ssh -N -L 10250:/tmp/fan_controller.sock root@10.55.0.1).
+Override with FAN_CONTROLLER_ENDPOINT: "unix:/path" or "host:port".
 """
 import json
 import os
 import socket
 import sys
 
-SOCK = os.environ.get("FAN_CONTROLLER_SOCK", "/tmp/fan_controller.sock")
+UNIX_PATH = "/tmp/fan_controller.sock"
+TUNNEL_HOST, TUNNEL_PORT = "127.0.0.1", 10250
 STEPS = [25, 50, 75, 100]
 
 
+def make_socket():
+    endpoint = os.environ.get("FAN_CONTROLLER_ENDPOINT")
+    if endpoint is None:
+        if os.path.exists(UNIX_PATH):
+            endpoint = f"unix:{UNIX_PATH}"
+        else:
+            endpoint = f"{TUNNEL_HOST}:{TUNNEL_PORT}"
+    if endpoint.startswith("unix:"):
+        s = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+        s.connect(endpoint[len("unix:"):])
+    else:
+        host, port = endpoint.rsplit(":", 1)
+        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+        s.connect((host, int(port)))
+    return s
+
+
 def cmd(command: str) -> str:
-    with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as s:
+    with make_socket() as s:
         s.settimeout(2)
-        s.connect(SOCK)
         s.sendall(command.encode() + b"\n")
         chunks = []
         while True: