Add Windows support

Windows joins the three systems as its own entry in each table: DirectShow
through ffmpeg for capture, the Win32 clipboard and SendInput for the paste,
RegisterHotKey for the global shortcut, and the whisper.cpp and llama.cpp
Windows zips (the OpenBLAS whisper build, which transcribes about twice as
fast on a plain CPU). Settings go to APPDATA, data to LOCALAPPDATA, and
install.ps1 adds the Start Menu entry, the dikte command and an optional
autostart. Meetings are not supported yet: Windows offers nothing to record
the far side from.

Porting surfaced three fixes that were not Windows specific:

- A stopped or overlong download tried to delete its .part file while still
  holding it open, which Windows refuses. The unlinks now wait for the handle.
- The CLI transcribed files without handing the local servers their settings
  first, so a local provider failed with "no model downloaded" wherever the
  GUI had not run in the same process.
- The audio content types are pinned instead of asked of the registry, which
  answers differently machine to machine.

One fix is Windows specific but sits in shared code: shutdown() does not end
a blocked recv there, so stopping a request also closes the socket handle.

Co-Authored-By: Claude Fable 5 <[email protected]>

ipc.py

@@ -14,7 +14,9 @@ import sys
 
 from PyQt6.QtNetwork import QLocalSocket
 
-SERVER_NAME = "dikte-" + str(os.getuid())
+SERVER_NAME = "dikte-" + (
+    str(os.getuid()) if hasattr(os, "getuid")
+    else os.environ.get("USERNAME", "user"))
 
 # Long enough for a process that is already running to answer, short enough that
 # "nothing is running" is not a noticeable pause in front of a key press.