extras: add PowerShell greeting port, Rust gm-switch and Windows installers

- powershell_greeting: PowerShell port of fish_greeting (fastfetch random logo, sixel/kitty-direct per terminal, logs, modes)

- win-gm-switch: Rust port of gm-switch.cpp (cross-platform mode file, --status, Start Menu shortcuts installer)

- install-windows.ps1 umbrella installer; README pointers

extras/powershell_greeting/config.example.ps1

@@ -0,0 +1,24 @@
+# Copy to %APPDATA%\j621Greeting\config.ps1 and adjust.
+# This file runs every time the greeting starts; an environment variable that
+# is already set in your session wins over the values here.
+
+# API origin (no trailing slash needed).
+if (-not $env:J621_BASE) { $env:J621_BASE = 'https://j621.rainbow-herring.ts.net' }
+
+# Web origin used in the printed link. Only needed when the SPA is served
+# from a different host than the API (split deployment).
+# if (-not $env:J621_WEB) { $env:J621_WEB = 'https://j621-frontend.rainbow-herring.ts.net' }
+
+# API token. Optional: gives you signed image URLs and access to items that
+# are hidden from guests. The recommended value is a scoped greeting token
+# (Account -> Shell tokens, or /tokens in the app): those only work with
+# /api/random/, so they are safe to keep in this file. The full API token
+# works too, but grants everything.
+# if (-not $env:J621_TOKEN) { $env:J621_TOKEN = 'j621r_paste-your-token-here' }
+
+# fastfetch config name used for the greeting logo.
+# if (-not $env:J621_FASTFETCH_CONFIG) { $env:J621_FASTFETCH_CONFIG = 'jake' }
+
+# Force a fastfetch logo type instead of the per-terminal default
+# (Windows Terminal -> sixel, WezTerm -> kitty-direct, others -> no image).
+# if (-not $env:J621_LOGO_TYPE) { $env:J621_LOGO_TYPE = 'kitty-direct' }