feat: chat language display, disabled EntitySelect items, dev scripts

Chat language:
- Added language_code field to TelegramChat model + migration
- Saved from message.from.language_code on webhook/polling
- Displayed as badge on bot chat cards and target receiver items
- Resolved from DB in target API response (works for existing receivers)
- Shown in chat picker dropdown (desc includes language)

EntitySelect improvements:
- Tracker-target link selector shows all targets, already-linked ones
  appear disabled with "Already linked" hint
- Receiver chat picker shows already-added chats as disabled

Dev scripts:
- scripts/restart-backend.sh and restart-frontend.sh
- Updated .claude/docs/dev-servers.md to reference scripts

frontend/src/routes/notification-trackers/+page.svelte

@@ -429,7 +429,7 @@
 						{tracker}
 						{trackingConfigs}
 						{templateConfigs}
-						unlinkedTargets={getUnlinkedTargets(tracker)}
+						unlinkedTargets={targets}
 						newLinkTargetId={newLinkTargetId[tracker.id] || 0}
 						newLinkTrackingConfigId={newLinkTrackingConfigId[tracker.id] || 0}
 						newLinkTemplateConfigId={newLinkTemplateConfigId[tracker.id] || 0}

frontend/src/routes/notification-trackers/LinkedTargetsSection.svelte

@@ -57,11 +57,14 @@
 
 	const trackingConfigItems = $derived(toItems(trackingConfigs));
 	const templateConfigItems = $derived(toItems(templateConfigs));
+	const linkedTargetIds = $derived(new Set((tracker.tracker_targets || []).map((tt: any) => tt.target_id)));
 	const targetItems = $derived<EntityItem[]>(unlinkedTargets.map(tgt => ({
 		value: tgt.id,
 		label: tgt.name,
 		icon: tgt.icon || (tgt.type === 'telegram' ? 'mdiSend' : 'mdiWebhook'),
 		desc: tgt.type,
+		disabled: linkedTargetIds.has(tgt.id),
+		disabledHint: linkedTargetIds.has(tgt.id) ? t('notificationTracker.alreadyLinked') : undefined,
 	})));
 </script>