feat: default tracker configs, email validation, expandable target links

- Tracker now has default_tracking_config_id and default_template_config_id
  that apply to all linked targets unless overridden per-target
- Dispatch falls back to tracker defaults when per-link configs are null
- Email bot creation validates SMTP connection before saving
- Email notifications sent as HTML (links render properly)
- Linked target items are expandable: collapsed shows config CrossLinks,
  expanded shows config selectors; action buttons always visible
- Fix email bot test button icon (mdiEmailSend → mdiSend)
- Fix target type icons in LinkedTargetsSection for all types
- Provider filter moved above search in sidebar

frontend/src/lib/i18n/en.json

@@ -207,6 +207,8 @@
 		"quietHoursStart": "Quiet hours start",
 		"quietHoursEnd": "Quiet hours end",
 		"batchDuration": "Batch duration (seconds)",
+		"defaultTrackingConfig": "Default tracking config",
+		"defaultTemplateConfig": "Default template config",
 		"linkedTargets": "targets",
 		"noLinkedTargets": "No targets linked. Add a target below.",
 		"addTarget": "Add target",
@@ -540,7 +542,8 @@
 		"albumFields": "Album fields (in {% for album in albums %})",
 		"confirmDelete": "Delete this template config?",
 		"invalidFormat": "Invalid format string",
-		"filterSlots": "Filter slots..."
+		"filterSlots": "Filter slots...",
+		"slots": "slots"
 	},
 	"templateVars": {
 		"message_assets_added": {
@@ -645,6 +648,8 @@
 		"templateConfig": "Controls the message format. Uses default templates if not set.",
 		"scanInterval": "How often to poll the provider for changes, in seconds. Lower = faster detection but more API calls.",
 		"batchDuration": "Time to accumulate changes before dispatching notifications. 0 = send immediately.",
+		"defaultTrackingConfig": "Applied to all linked targets unless overridden per target.",
+		"defaultTemplateConfig": "Applied to all linked targets unless overridden per target.",
 		"defaultCount": "How many results to return when the user doesn't specify a count (1-20).",
 		"responseMode": "Media: send actual photos. Text: send filenames/links only. Media mode uses more bandwidth.",
 		"botLocale": "Language for command descriptions in Telegram's menu and bot response messages.",