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/targets/+page.svelte

@@ -475,7 +475,7 @@
 							{#if target.type === 'broadcast' && target.child_targets?.length}
 								<span class="text-xs px-1.5 py-0.5 rounded bg-[var(--color-muted)] text-[var(--color-muted-foreground)]">{target.child_targets.length} {t('targets.childTargets')}</span>
 							{:else if target.type !== 'broadcast' && (target.receivers || []).length > 0}
-								<span class="text-xs px-1.5 py-0.5 rounded bg-[var(--color-muted)] text-[var(--color-muted-foreground)]">{(target.receivers || []).length} receiver(s)</span>
+								<span class="text-xs px-1.5 py-0.5 rounded bg-[var(--color-muted)] text-[var(--color-muted-foreground)]">{(target.receivers || []).length} {t('targets.receivers')}</span>
 							{/if}
 							{#if getBotName(target)}<CrossLink href={getBotHref(target)} icon="mdiRobot" label={getBotName(target) ?? ''} entityId={getBotEntityId(target)} />{/if}
 						</div>