Add test suite (63 tests) and CI workflow, fix Settings API bugs

Test suite covering all three layers:

Python backend (25 tests):
- AppController: state machine, start/stop, callbacks, settings reload
- API server: REST endpoints, config CRUD, status, devices
- Config: dot-notation get/set, persistence, nested paths
- Main headless: ready event port format validation

Svelte frontend (14 tests via Vitest):
- Backend store: exported properties/methods, port derivation, URLs
- Config store: method names (fetchConfig not loadConfig), defaults
- Transcriptions store: add/clear/plaintext
- File extension regression: ensures $state runes only in .svelte.ts

Rust sidecar (24 tests via cargo test):
- Platform/arch detection, asset name construction
- Ready event deserialization (with extra fields tolerance)
- Path construction, version read/write, old version cleanup
- Zip extraction, SidecarManager lifecycle

CI workflow (.gitea/workflows/test.yml):
- Runs on push to main and PRs
- Three parallel jobs: Python, Frontend, Rust

Also fixes three bugs found during test planning:
- Settings: /api/check-updates -> GET /api/check-update
- Settings: /api/remote/login -> /api/login
- Settings: /api/remote/register -> /api/register

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

src/lib/stores/config.test.ts

@@ -0,0 +1,48 @@
+import { describe, it, expect, vi } from "vitest";
+
+// Mock fetch so the backend store module doesn't make real requests
+vi.stubGlobal(
+  "fetch",
+  vi.fn(() =>
+    Promise.resolve({
+      ok: true,
+      json: () => Promise.resolve({}),
+    })
+  )
+);
+
+// Mock WebSocket for the backend store dependency
+class MockWebSocket {
+  onopen: ((ev: Event) => void) | null = null;
+  onclose: ((ev: CloseEvent) => void) | null = null;
+  onmessage: ((ev: MessageEvent) => void) | null = null;
+  onerror: ((ev: Event) => void) | null = null;
+  close = vi.fn();
+}
+vi.stubGlobal("WebSocket", MockWebSocket);
+
+import { configStore } from "./config.svelte.ts";
+
+describe("config store", () => {
+  it("test_has_fetchConfig_method", () => {
+    expect(typeof configStore.fetchConfig).toBe("function");
+  });
+
+  it("test_has_updateConfig_method", () => {
+    expect(typeof configStore.updateConfig).toBe("function");
+  });
+
+  it("test_config_defaults_have_expected_keys", () => {
+    const cfg = configStore.config;
+    expect(cfg).toHaveProperty("user");
+    expect(cfg).toHaveProperty("audio");
+    expect(cfg).toHaveProperty("transcription");
+    expect(cfg).toHaveProperty("display");
+    expect(cfg).toHaveProperty("remote");
+    expect(cfg).toHaveProperty("updates");
+  });
+
+  it("test_remote_config_has_byok_api_key", () => {
+    expect(configStore.config.remote.byok_api_key).toBeDefined();
+  });
+});