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/requirements.txt

@@ -4,3 +4,7 @@ pyautogui>=0.9.54
 pyobjc-core>=10.0; sys_platform == "darwin"
 pyobjc-framework-Quartz>=10.0; sys_platform == "darwin"
 pyobjc-framework-Cocoa>=10.0; sys_platform == "darwin"
+# Linux: pyautogui drives X11 through Xlib. Raising the browser window is done
+# with xdotool, which is an apt package rather than a wheel - setup-linux.sh
+# installs it.
+python-xlib>=0.33; sys_platform == "linux"