feat(v3): migrate workspace to API, Tauri desktop, and v3 crates; remove legacy MCP stack

- Add apps/api, desktop Tauri shell, domain/application/crypto/device-auth/infrastructure-db
- Replace desktop-daemon vault integration; drop secrets-core and secrets-mcp*
- Ignore apps/desktop/dist and generated Tauri icons; document icon/dist steps in AGENTS.md
- Apply rustfmt; fix clippy (collapsible_if, HTTP method as str)

.gitignore

@@ -6,4 +6,13 @@
 tmp/
 client_secret_*.apps.googleusercontent.com.json
 node_modules/
-*.pyc
+*.pyc
+
+# Desktop: Tauri frontend bundle (tauri.conf.json build.frontendDist)
+apps/desktop/dist/
+
+# Tauri app icon pack: generated by `cargo tauri icon apps/desktop/src-tauri/icons/icon.png`
+# Version control only the 1024×1024 master; regenerate the rest locally or in release builds.
+apps/desktop/src-tauri/icons/**
+!apps/desktop/src-tauri/icons/
+!apps/desktop/src-tauri/icons/icon.png