feat: add Windows right-click "Unpack with fsdecrypt" context menu

install-context-menu.bat / uninstall-context-menu.bat add a per-user (HKCU)
Explorer entry for .app and .opt files, with an embedded unpack icon
(assets/fsdecrypt.ico). No admin needed; the normal double-click association is
untouched. The registered command keeps the console open after extraction.
Documented in the README.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -50,6 +50,27 @@ fsdecrypt ABCD_1.00.00_20240101120000_0.app ABCD_1.01.00_20240215143000_1_1.00.0
 
 The output folder is named after the input file (e.g. `ABCD_1.01.00_20240215143000_1_1.00.00/`).
 
+## Windows Right-Click Menu
+
+Two batch files add an **"Unpack with fsdecrypt"** entry (with an unpack icon)
+to the right-click menu of `.app` and `.opt` files:
+
+- **`install-context-menu.bat`** — install the menu entry
+- **`uninstall-context-menu.bat`** — remove it
+
+Just double-click `install-context-menu.bat`. No administrator rights are
+needed — it installs under the current user only (`HKCU`) and leaves the normal
+double-click association untouched. Right-clicking a file then runs the full
+extraction and keeps the console window open until you press a key.
+
+`install-context-menu.bat` finds `fsdecrypt.exe` automatically when run from the
+repo root (it looks in `target\release` then `target\debug`) or when placed next
+to the binary. The icon is taken from `assets\fsdecrypt.ico` (or `fsdecrypt.ico`
+beside the script). The menu stores the absolute path it found, so re-run it if
+you move the exe.
+
+> On Windows 11 the entry appears under **Show more options** (the classic menu).
+
 ## External Key Files
 
 For games not in the built-in key database, place a file named `{GAME_ID}.bin` in the working directory: