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)

crates/device-auth/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "secrets-device-auth"
+version = "0.1.0"
+edition.workspace = true
+
+[lib]
+name = "secrets_device_auth"
+path = "src/lib.rs"
+
+[dependencies]
+anyhow.workspace = true
+hex.workspace = true
+rand.workspace = true
+sha2.workspace = true
+url.workspace = true
+uuid.workspace = true

crates/device-auth/src/lib.rs

@@ -0,0 +1,27 @@
+use anyhow::{Context, Result};
+use rand::{Rng, RngExt};
+use sha2::{Digest, Sha256};
+use url::Url;
+
+pub fn loopback_redirect_uri(port: u16) -> Result<Url> {
+    Url::parse(&format!("http://127.0.0.1:{port}/oauth/callback"))
+        .context("failed to build loopback redirect URI")
+}
+
+pub fn new_device_fingerprint() -> String {
+    let mut bytes = [0_u8; 16];
+    rand::rng().fill(&mut bytes);
+    hex::encode(bytes)
+}
+
+pub fn new_device_login_token() -> String {
+    let mut bytes = [0_u8; 32];
+    rand::rng().fill_bytes(&mut bytes);
+    hex::encode(bytes)
+}
+
+pub fn hash_device_login_token(token: &str) -> String {
+    let mut hasher = Sha256::new();
+    hasher.update(token.as_bytes());
+    hex::encode(hasher.finalize())
+}