fix(dashboard): guard failureThreshold against empty/invalid settings values

v-model.number on the Settings input returns an empty string when cleared,
which persists through the PATCH and then coerces error_count >= '' to
error_count >= 0 — flagging every source as failing. Coerce to Number and
fall back to 5 unless the value is a finite number >= 1.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/views/Dashboard.vue

@@ -354,10 +354,11 @@ let refreshInterval = null
 const stats = computed(() => downloadsStore.stats)
 const recentActivity = computed(() => downloadsStore.recentActivity)
 
-// Read threshold from settings, default 5 if unset
-const failureThreshold = computed(() =>
-  settingsStore.settings['dashboard.failure_threshold'] ?? 5
-)
+// Read threshold from settings, default 5 if unset or invalid
+const failureThreshold = computed(() => {
+  const n = Number(settingsStore.settings['dashboard.failure_threshold'])
+  return Number.isFinite(n) && n >= 1 ? n : 5
+})
 
 // Per-platform health for platforms with ≥1 enabled source
 const platformHealth = computed(() => {