Add Linux/Raspberry Pi setup and port the clicker to X11

The clicker had no Linux support at all: pygetwindow has no X11 backend, so
_other_activate returned "window management is unsupported" and every step
failed before it could click. focus.py now has a third backend that raises the
browser with xdotool and reads the focused window's WM_CLASS to verify it came
forward - the same activate-then-confirm shape as the macOS and Windows paths.

setup-linux.sh mirrors setup-windows.bat with apt instead of winget. Three
things are specific to this platform rather than incidental:

- Node comes from NodeSource. Pi OS ships one too old for Next 16, and the LTS
  line is also what better-sqlite3 publishes prebuilt arm64 binaries for.
- Python dependencies go in a virtualenv. Pi OS Bookworm enforces PEP 668, so
  pip into the system interpreter fails with externally-managed-environment.
- Autostart uses an XDG ~/.config/autostart entry, the direct analogue of the
  Windows Startup folder: no sudo, and it runs inside the graphical session,
  which the clicker needs for DISPLAY.

Wayland is called out in four places - the session guard, diagnose.py, the
installer and both READMEs - because Pi OS on a Pi 5 defaults to it and the
clicker simply cannot work there. Wayland does not let one client synthesise
input into another, so this is a switch-to-X11 situation, not a bug to fix.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

clicker/diagnose.py

@@ -12,6 +12,8 @@ OS after each move, so the answer doesn't depend on anything being rendered.
 Nothing is clicked. The cursor is moved and put back where it started.
 """
 
+import os
+import shutil
 import sys
 import time
 
@@ -27,6 +29,19 @@ def check_platform() -> None:
     line("os", sys.platform)
     line("python", sys.version.split()[0])
 
+    if sys.platform.startswith("linux"):
+        # The two things that decide whether the clicker can work at all here.
+        session = os.environ.get("XDG_SESSION_TYPE", "unknown")
+        display = os.environ.get("DISPLAY") or "(unset)"
+        line("session type", session)
+        line("DISPLAY", display)
+        line("xdotool", shutil.which("xdotool") or "NOT INSTALLED — sudo apt install -y xdotool")
+        if session == "wayland" or (os.environ.get("WAYLAND_DISPLAY") and not os.environ.get("DISPLAY")):
+            print("     Wayland cannot be automated: xdotool and pyautogui both speak X11,")
+            print("     and Wayland does not let one client drive another. On Raspberry Pi OS:")
+            print("     sudo raspi-config -> Advanced Options -> Wayland -> X11, then reboot.")
+        return
+
     if sys.platform != "win32":
         return