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/README.md

@@ -53,6 +53,17 @@ VS Code — under System Settings → Privacy & Security → Accessibility. With
 `pyautogui` moves nothing and fails silently, which looks identical to a bad
 selector.
 
+### Linux (including Raspberry Pi)
+
+`setup-linux.sh` installs `xdotool` and `scrot` and builds a virtualenv for the
+Python side.
+
+**X11 only.** pygetwindow has no X11 backend, so raising the browser goes
+through `xdotool` instead. Neither that nor pyautogui works under Wayland — it
+refuses by design to let one client drive another — and Raspberry Pi OS on a Pi
+5 defaults to Wayland. Switch with `sudo raspi-config` -> Advanced Options ->
+Wayland -> X11 and reboot. `diagnose.py` prints which session you are in.
+
 ### Windows
 
 No extra permissions, but two things differ.