Remove npm build dependency

[enaboapps]

Summary

• remove npm/package files and Node packaging scripts
• replace C# packaging and package verification with PowerShell scripts
• move package output to dist/
• update CI and release workflow commands away from npm
• update README/AGENTS for the C# app version source and PowerShell package flow

Closes #285.

Validation

• dotnet restore src/SwitchifyPc.sln
• dotnet build src/SwitchifyPc.sln -c Release --no-restore
• dotnet test src/SwitchifyPc.sln -c Release --no-build
• powershell -NoProfile -ExecutionPolicy Bypass -File scripts\Package-Windows.ps1 -StageOnly -SkipSign
• powershell -NoProfile -ExecutionPolicy Bypass -File scripts\Verify-DotnetPackage.ps1
• rg -n --hidden npm|package.json|package-lock|node_modules|\.cjs|electron|Electron|electron-builder|electron-vite|vitest|vite|react|dist-dotnet . -g !.git/** -g !dist/** -g !src/**/bin/** -g !src/**/obj/**

Local note: full unsigned installer packaging reaches NSIS lookup and stops because makensis.exe is not installed on this machine. The signing runner/package environment is expected to provide NSIS or SWITCHIFY_MAKENSIS_EXE.