feat(desktop): embed BoosAPI Desktop client as a subproject

Vendor the rebranded BoosAPI Desktop client (Tauri + Vue) into metapi as
a monorepo subdirectory under desktop/. Source only — node_modules,
dist, Cargo target and the upstream .git history are excluded.

desktop/ keeps its own independent Cargo workspace (desktop/src-tauri
with crates/*); it is not part of the backend boosapi crate, so a
`cargo check` at the metapi root still compiles only the backend. Build
the desktop app from desktop/src-tauri per desktop/README.md.

Note: app icons are still placeholder assets pending the final BoosAPI
logo. The updater signing keys live outside the repo and are not
included.

desktop/src-tauri/crates/providers/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "boosapi-providers"
+version = "0.0.1"
+edition = "2021"
+
+[dependencies]
+boosapi-core = { path = "../core" }
+serde = { workspace = true }
+serde_json = { workspace = true }
+tokio = { workspace = true }
+reqwest = { version = "0.12", features = ["json", "stream", "socks", "rustls-tls", "multipart"] }
+futures = "0.3"
+async-trait = "0.1"
+tracing = { workspace = true }
+tokio-tungstenite = { version = "0.26", features = ["native-tls"] }
+base64 = "0.22"
+image = { version = "0.25", default-features = false, features = ["png", "jpeg", "webp"] }