fix: comprehensive API/UI review — 26 bug fixes and improvements

Backend:
- Scheduler lifecycle sync: create/update/delete tracker now syncs APScheduler jobs
- Test-periodic/test-memory endpoints render actual Jinja2 templates with sample data
- Cascade cleanup on tracker delete (TrackerState removed, EventLog nullified)
- Fix user_id=0 FK violation for system-owned TemplateConfig (removed FK constraint)
- Fix API key leak: only attach x-api-key header for internal provider URLs
- Validate config ownership in tracker_targets create/update
- Fix _response() double-emit of created_at in template/tracking configs
- Add per-target-link test endpoints (test, test-periodic, test-memory)

Frontend:
- Fix orphaned provider on test exception in providers/new
- Add submitting guard + disabled state to targets save button
- Move test buttons from tracker card to per-target-link rows
- Fix Svelte 5 async $state reactivity (spread reassignment for all Record mutations)
- i18n for dashboard timeAgo and event type badges (EN + RU)
- Add required attribute to chat select dropdown in targets
- Fix font CSS vars to prioritize imported DM Sans / JetBrains Mono
- Standardize empty states with centered icon + text across all 6 list pages
- Add stagger-children animation class to all list containers
- Fix slide transition duration consistency (200ms everywhere)
- Standardize border-radius to rounded-md across all form inputs
- Fix providers/new page structure (h2 + mb-8 spacing)
- Fix tracker card action row overflow (flex-wrap justify-end)
- JinjaEditor dark mode reactivity (recreate editor on theme change)
- Add aria-labels to mobile nav items
- Make ConfirmModal confirm button label/icon configurable
- Remove double error reporting on providers page
- Add telegram bot edit functionality (name editing via PUT)
- i18n for External Domain label on provider forms

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/routes/+layout.svelte

@@ -229,13 +229,13 @@
 		<!-- Mobile bottom nav -->
 		<nav class="mobile-nav" style="position: fixed; bottom: 0; left: 0; right: 0; z-index: 50; background: var(--color-sidebar); border-top: 1px solid var(--color-border); display: none; justify-content: space-around; padding: 0.375rem 0; backdrop-filter: blur(12px);">
 			{#each navItems.slice(0, 5) as item}
-				<a href={item.href}
+				<a href={item.href} aria-label={t(item.key)}
 					class="flex flex-col items-center gap-0.5 px-2 py-1.5 text-xs rounded-lg transition-all duration-200"
 					style="color: {isActive(item.href) ? 'var(--color-primary)' : 'var(--color-muted-foreground)'};">
 					<MdiIcon name={item.icon} size={20} />
 				</a>
 			{/each}
-			<button onclick={logout}
+			<button onclick={logout} aria-label={t('nav.logout')}
 				class="flex flex-col items-center gap-0.5 px-2 py-1.5 text-xs" style="color: var(--color-muted-foreground);">
 				<MdiIcon name="mdiLogout" size={20} />
 			</button>