refactor: replace NSIS with Inno Setup for Windows installer

Switch from NSIS (early 2000s) to Inno Setup 6 — the modern standard
used by VS Code and most Electron apps. Features:

- Modern wizard UI (WizardStyle=modern)
- LZMA2/ultra64 solid compression (138.9 MB, down from 143 MB with NSIS)
- Per-user install to %LOCALAPPDATA%\Programs\Aryx (no admin needed)
- Optional desktop shortcut, Start Menu group
- Auto-closes running instance before install/uninstall
- Proper Add/Remove Programs integration via AppId
- Launch-after-install option

CI updated to install Inno Setup via Chocolatey on windows-latest.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

assets/installer/windows.iss

@@ -0,0 +1,85 @@
+; Inno Setup script for Aryx
+; Metadata values are passed via /D preprocessor defines from the build script.
+
+#ifndef PRODUCT_NAME
+  #define PRODUCT_NAME "Aryx"
+#endif
+#ifndef PRODUCT_VERSION
+  #define PRODUCT_VERSION "0.0.0"
+#endif
+#ifndef PRODUCT_PUBLISHER
+  #define PRODUCT_PUBLISHER "David Kaya"
+#endif
+#ifndef SOURCE_DIR
+  #error "SOURCE_DIR must be defined (path to the packaged app directory)."
+#endif
+#ifndef OUTPUT_DIR
+  #error "OUTPUT_DIR must be defined (path to the output directory)."
+#endif
+#ifndef OUTPUT_FILENAME
+  #error "OUTPUT_FILENAME must be defined (output installer filename without extension)."
+#endif
+
+#ifndef ICON_PATH
+  #define ICON_PATH SOURCE_DIR + "\" + PRODUCT_NAME + ".exe"
+#endif
+
+[Setup]
+AppId={{B8A3E7F1-4D2C-4A9B-8E6F-1C3D5A7B9E0F}
+AppName={#PRODUCT_NAME}
+AppVersion={#PRODUCT_VERSION}
+AppPublisher={#PRODUCT_PUBLISHER}
+AppSupportURL=https://github.com/davidkaya/aryx
+DefaultDirName={localappdata}\Programs\{#PRODUCT_NAME}
+DefaultGroupName={#PRODUCT_NAME}
+PrivilegesRequired=lowest
+OutputDir={#OUTPUT_DIR}
+OutputBaseFilename={#OUTPUT_FILENAME}
+Compression=lzma2/ultra64
+SolidCompression=yes
+SetupIconFile={#ICON_PATH}
+UninstallDisplayIcon={app}\{#PRODUCT_NAME}.exe
+WizardStyle=modern
+DisableProgramGroupPage=yes
+CloseApplications=force
+RestartApplications=no
+
+[Languages]
+Name: "english"; MessagesFile: "compiler:Default.isl"
+
+[Files]
+Source: "{#SOURCE_DIR}\*"; DestDir: "{app}"; Flags: ignoreversion recursesubdirs createallsubdirs
+
+[Icons]
+Name: "{group}\{#PRODUCT_NAME}"; Filename: "{app}\{#PRODUCT_NAME}.exe"
+Name: "{autodesktop}\{#PRODUCT_NAME}"; Filename: "{app}\{#PRODUCT_NAME}.exe"; Tasks: desktopicon
+
+[Tasks]
+Name: "desktopicon"; Description: "{cm:CreateDesktopIcon}"; GroupDescription: "{cm:AdditionalIcons}"; Flags: unchecked
+
+[Run]
+Filename: "{app}\{#PRODUCT_NAME}.exe"; Description: "{cm:LaunchProgram,{#PRODUCT_NAME}}"; Flags: nowait postinstall skipifsilent
+
+[UninstallDelete]
+Type: filesandordirs; Name: "{app}"
+
+[Code]
+procedure CurStepChanged(CurStep: TSetupStep);
+var
+  ResultCode: Integer;
+begin
+  if CurStep = ssInstall then
+  begin
+    Exec('taskkill', '/F /IM {#PRODUCT_NAME}.exe', '', SW_HIDE, ewWaitUntilTerminated, ResultCode);
+  end;
+end;
+
+procedure CurUninstallStepChanged(CurUninstallStep: TUninstallStep);
+var
+  ResultCode: Integer;
+begin
+  if CurUninstallStep = usUninstall then
+  begin
+    Exec('taskkill', '/F /IM {#PRODUCT_NAME}.exe', '', SW_HIDE, ewWaitUntilTerminated, ResultCode);
+  end;
+end;