feat(translation): "Test connection" button — on-demand Interpreter health check (#143)

New POST /api/settings/translation/test pings /v1/health for a GIVEN base URL (not
the saved one), so the operator can verify a URL before enabling it. TranslationCard
gains a Test-connection button that reports reachable/unreachable inline and
updates the status dot. Endpoint test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/api/settings.py

@@ -325,6 +325,19 @@ async def translation_status():
     })
 
 
+@settings_bp.route("/settings/translation/test", methods=["POST"])
+async def translation_test():
+    """On-demand reachability check for a GIVEN Interpreter base URL (the Settings
+    'Test connection' button) — pings /v1/health without saving, so the operator
+    can verify a URL before enabling. Health runs in a thread (sync client)."""
+    body = await request.get_json()
+    base_url = ""
+    if isinstance(body, dict):
+        base_url = (body.get("base_url") or "").strip()
+    healthy = await asyncio.to_thread(ic.health, base_url) if base_url else False
+    return jsonify({"healthy": healthy})
+
+
 @settings_bp.route("/settings/translation/run", methods=["POST"])
 async def translation_run():
     """Enqueue the translate sweep now (the Settings 'Translate now' button)."""