Add speech-to-text feature using Faster Whisper container

Adds a mic button to the terminal UI that captures speech, transcribes
it via a Faster Whisper sidecar container, and injects the text into
the terminal input. Includes settings panel for model selection
(tiny/small/medium), port config, and container lifecycle management.

- stt-container/: Dockerfile + FastAPI server for Whisper transcription
- Rust backend: STT container management, transcribe_audio IPC command
- Frontend: useSTT hook, SttButton, SttSettings, WAV encoder
- CI: Gitea Actions workflow for multi-arch STT image builds

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/src/lib/wav.ts

@@ -0,0 +1,40 @@
+/**
+ * Encode PCM Int16 samples into a WAV file blob.
+ * Assumes mono channel at the given sample rate.
+ */
+export function encodeWav(samples: Int16Array, sampleRate: number): Blob {
+  const byteLength = samples.length * 2;
+  const buffer = new ArrayBuffer(44 + byteLength);
+  const view = new DataView(buffer);
+
+  // RIFF header
+  writeString(view, 0, "RIFF");
+  view.setUint32(4, 36 + byteLength, true);
+  writeString(view, 8, "WAVE");
+
+  // fmt chunk
+  writeString(view, 12, "fmt ");
+  view.setUint32(16, 16, true); // chunk size
+  view.setUint16(20, 1, true); // PCM format
+  view.setUint16(22, 1, true); // mono
+  view.setUint32(24, sampleRate, true);
+  view.setUint32(28, sampleRate * 2, true); // byte rate
+  view.setUint16(32, 2, true); // block align
+  view.setUint16(34, 16, true); // bits per sample
+
+  // data chunk
+  writeString(view, 36, "data");
+  view.setUint32(40, byteLength, true);
+
+  // PCM samples
+  const output = new Int16Array(buffer, 44);
+  output.set(samples);
+
+  return new Blob([buffer], { type: "audio/wav" });
+}
+
+function writeString(view: DataView, offset: number, str: string) {
+  for (let i = 0; i < str.length; i++) {
+    view.setUint8(offset + i, str.charCodeAt(i));
+  }
+}