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/public/audio-processor.js

@@ -0,0 +1,19 @@
+// AudioWorkletProcessor for PCM16 encoding (Float32 → Int16)
+class AudioPcm16Processor extends AudioWorkletProcessor {
+  process(inputs) {
+    const input = inputs[0];
+    if (!input || !input[0]) return true;
+
+    const float32 = input[0];
+    const int16 = new Int16Array(float32.length);
+    for (let i = 0; i < float32.length; i++) {
+      const s = Math.max(-1, Math.min(1, float32[i]));
+      int16[i] = s < 0 ? s * 0x8000 : s * 0x7fff;
+    }
+
+    this.port.postMessage(int16.buffer, [int16.buffer]);
+    return true;
+  }
+}
+
+registerProcessor('audio-pcm16-processor', AudioPcm16Processor);