Document Windows support and the AutoBuyer components

focus.py now verifies on Windows rather than assuming activation worked. Windows
declines to raise a window for a process that doesn't own the foreground — it
flashes the taskbar and the call returns as if it succeeded — so the runner reads
the foreground window's process back and reports a failure instead of clicking
into a background window. Same gap that was fixed on macOS earlier.

The runner also declares itself DPI-aware at startup. Without it Windows reports
a virtualised screen size and rescales the coordinates it accepts, while the
browser keeps reporting CSS pixels; on a display at 125% or 150% the two disagree
and clicks drift further off the further they are from the top-left.

Browser windows are matched on the owning process rather than the window title, so
an editor with chrome.js open is no longer mistaken for the browser. Linux now says
window management is unsupported there, rather than reporting no browser found —
pygetwindow has no X11 backend, and "no browser window" reads like Chrome is shut.

The main README gained a section on the AutoBuyer: what the three pieces are, how
to load the extension, and that neither the extension nor the runner reloads
itself when the source changes. That last point has been the cause of most of the
confusing failures so far, so it is called out in Updating too.

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

clicker/clicker.py

@@ -174,6 +174,11 @@ def main() -> int:
                   file=sys.stderr)
             return 1
 
+        # Before anything asks the OS how big the screen is. No-op off Windows.
+        dpi = focus.enable_dpi_awareness()
+        if dpi:
+            print(f"  {dpi}")
+
     # ── resolve and vet the text before touching anything ───────────────────
     text = ""
     if args.action == "type":