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/routes/notification-trackers/TrackerForm.svelte

@@ -16,11 +16,15 @@
 			collection_ids: string[];
 			scan_interval: number;
 			batch_duration: number;
+			default_tracking_config_id: number;
+			default_template_config_id: number;
 			filters: Record<string, any>;
 		};
 		providerItems: { value: number; label: string; icon: string; desc: string }[];
 		collections: any[];
 		collectionFilter?: string;
+		trackingConfigItems?: { value: number; label: string; icon: string }[];
+		templateConfigItems?: { value: number; label: string; icon: string }[];
 		editing: number | null;
 		submitting: boolean;
 		linkCheckLoading: boolean;
@@ -36,6 +40,8 @@
 		providerItems,
 		collections,
 		collectionFilter = $bindable(),
+		trackingConfigItems = [],
+		templateConfigItems = [],
 		editing,
 		submitting,
 		linkCheckLoading,
@@ -175,6 +181,24 @@
 		</div>
 		{/if}
 
+		<!-- Default configs -->
+		{#if trackingConfigItems.length > 0 || templateConfigItems.length > 0}
+		<div class="grid grid-cols-2 gap-3">
+			{#if trackingConfigItems.length > 0}
+			<div>
+				<label class="block text-sm font-medium mb-1">{t('notificationTracker.defaultTrackingConfig')}<Hint text={t('hints.defaultTrackingConfig')} /></label>
+				<EntitySelect items={[{value: 0, label: t('common.none'), icon: 'mdiMinus'}, ...trackingConfigItems]} bind:value={form.default_tracking_config_id} placeholder={t('common.none')} />
+			</div>
+			{/if}
+			{#if templateConfigItems.length > 0}
+			<div>
+				<label class="block text-sm font-medium mb-1">{t('notificationTracker.defaultTemplateConfig')}<Hint text={t('hints.defaultTemplateConfig')} /></label>
+				<EntitySelect items={[{value: 0, label: t('common.none'), icon: 'mdiMinus'}, ...templateConfigItems]} bind:value={form.default_template_config_id} placeholder={t('common.none')} />
+			</div>
+			{/if}
+		</div>
+		{/if}
+
 		<button type="submit" disabled={submitting || linkCheckLoading} class="px-4 py-2 bg-[var(--color-primary)] text-[var(--color-primary-foreground)] rounded-md text-sm font-medium hover:opacity-90 disabled:opacity-50">
 			{#if linkCheckLoading}{t('notificationTracker.checkingLinks')}{:else}{editing ? t('common.save') : t('notificationTracker.createTracker')}{/if}
 		</button>