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)."""

frontend/src/components/settings/TranslationCard.vue

@@ -28,9 +28,17 @@
       @change="onSave"
     />
 
-    <div class="d-flex align-center mb-3" style="gap: 8px;">
+    <div class="d-flex align-center flex-wrap mb-3" style="gap: 8px;">
       <v-icon size="12" :color="statusColor">mdi-circle</v-icon>
       <span class="fc-muted text-body-2">{{ statusText }}</span>
+      <v-btn
+        size="x-small" variant="tonal" rounded="pill" class="ml-2"
+        :loading="testing" :disabled="!baseUrl" @click="onTest"
+      >Test connection</v-btn>
+      <span
+        v-if="testResult !== null" class="text-caption"
+        :class="testResult ? 'text-success' : 'text-error'"
+      >{{ testResult ? '✓ reachable' : '✗ unreachable' }}</span>
     </div>
 
     <div class="d-flex align-center flex-wrap" style="gap: 10px;">
@@ -67,6 +75,8 @@ const baseUrl = ref('')
 const targetLang = ref('en')
 const busy = ref(false)
 const running = ref(false)
+const testing = ref(false)
+const testResult = ref(null)   // null = not tested this session; true/false = last result
 const status = ref(null)
 const err = ref(null)
 
@@ -86,6 +96,25 @@ async function loadStatus() {
   catch { status.value = null }
 }
 
+async function onTest() {
+  // Ping /v1/health for the CURRENTLY-typed URL (not the saved one), so a new
+  // URL can be verified before enabling. Also reflects in the status dot.
+  testing.value = true
+  err.value = null
+  testResult.value = null
+  try {
+    const r = await api.post('/api/settings/translation/test', {
+      body: { base_url: baseUrl.value.trim() },
+    })
+    testResult.value = !!r.healthy
+    if (status.value) status.value.healthy = !!r.healthy
+  } catch (e) {
+    err.value = e.message
+  } finally {
+    testing.value = false
+  }
+}
+
 onMounted(async () => {
   try {
     await store.loadSettings()

tests/test_api_settings.py

@@ -69,6 +69,20 @@ async def test_translation_run_requires_config(client):
     assert resp.status_code == 400   # disabled / no base URL
 
 
+@pytest.mark.asyncio
+async def test_translation_test_connection(client, monkeypatch):
+    # Tests a GIVEN url (not the saved one) without persisting anything.
+    monkeypatch.setattr("backend.app.api.settings.ic.health", lambda *a, **k: True)
+    resp = await client.post(
+        "/api/settings/translation/test", json={"base_url": "http://i.lan"})
+    assert resp.status_code == 200
+    assert (await resp.get_json())["healthy"] is True
+    # Empty URL → False, no health call.
+    empty = await client.post(
+        "/api/settings/translation/test", json={"base_url": ""})
+    assert (await empty.get_json())["healthy"] is False
+
+
 @pytest.mark.asyncio
 async def test_translation_run_enqueues_when_configured(client, monkeypatch):
     monkeypatch.setattr(