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/bots/EmailBotTab.svelte

@@ -161,7 +161,7 @@
 						</div>
 					</div>
 					<div class="flex items-center gap-1">
-						<IconButton icon="mdiEmailSend" title={t('emailBot.testConnection')} onclick={() => testEmailBot(bot.id)} disabled={emailTesting[bot.id]} />
+						<IconButton icon="mdiSend" title={t('emailBot.testConnection')} onclick={() => testEmailBot(bot.id)} disabled={emailTesting[bot.id]} />
 						<IconButton icon="mdiPencil" title={t('common.edit')} onclick={() => editEmailBot(bot)} />
 						<IconButton icon="mdiDelete" title={t('common.delete')} onclick={() => removeEmail(bot.id)} variant="danger" />
 					</div>