feat: add app settings, vault plugin state, and first-run vault selection
- internal/core/appsettings/ — app settings manager (~/.config/verstak/config.json)
- internal/core/pluginstate/ — vault plugin state (.verstak/plugins.json)
- internal/api/app.go — Wails bindings for app settings + plugin state
- main.go — init app settings, auto-open vault, init plugin state, disabled plugin filtering
- Plugin state: enable/disable, desired plugins, missing-installed tracking
- App settings: currentVaultPath, recentVaults, theme, devMode, windowState