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>

package.json

@@ -12,16 +12,19 @@
   "devDependencies": {
     "@sveltejs/vite-plugin-svelte": "^5.0.0",
     "@tauri-apps/cli": "^2.0.0",
+    "@testing-library/svelte": "^5.3.1",
     "@tsconfig/svelte": "^5.0.0",
+    "jsdom": "^29.0.2",
     "svelte": "^5.0.0",
     "svelte-check": "^4.0.0",
     "typescript": "~5.6.0",
-    "vite": "^6.0.0"
+    "vite": "^6.0.0",
+    "vitest": "^4.1.3"
   },
   "dependencies": {
     "@tauri-apps/api": "^2.0.0",
     "@tauri-apps/plugin-dialog": "^2.0.0",
-    "@tauri-apps/plugin-shell": "^2.0.0",
-    "@tauri-apps/plugin-process": "^2.0.0"
+    "@tauri-apps/plugin-process": "^2.0.0",
+    "@tauri-apps/plugin-shell": "^2.0.0"
   }
 }