refactor: comprehensive consistency review — UI/UX, code quality, functional parity

Fix 19 issues across 3 priority tiers found during full-codebase review:

CRITICAL:
- Fix undefined --color-secondary CSS variable causing invisible UI elements
- Fix Google Photos command templates using nonexistent asset.originalFileName
- Fix scheduler template variable docs (tracker_name → schedule_name)
- Add missing admin guard on notification template update endpoint

HIGH:
- Fix 5 hardcoded English strings missing i18n (MultiEntitySelect, actions,
  settings, TelegramBotTab, users)
- Replace 17 raw <button> elements with shared <Button> component
- Replace 5 raw error divs with shared <ErrorBanner> component
- Refactor webhook handler duplication into shared _dispatch_webhook_event()
- Add 30+ provider-specific fields to TrackingConfig TypeScript type
- Add default TrackingConfig seeds for immich and google_photos
- Add provider-specific command variable docs (Gitea, Planka, NUT, GP, Webhook)

MEDIUM:
- Replace hardcoded hex colors and Tailwind classes with CSS variable tokens
- Remove dead code (unused imports, orphaned check_notification_tracker)
- Fix Svelte 5 patterns ($state for _prevProviderId, remove unnecessary as any)
- Fix inconsistent POST response shape (targets now returns full response)
- Fix Google Photos template dead asset.year branches, clarify album_url docs

frontend/src/routes/tracking-configs/+page.svelte

@@ -19,6 +19,8 @@
 	import { highlightFromUrl } from '$lib/highlight';
 	import { globalProviderFilter } from '$lib/stores/provider-filter.svelte';
 	import { getDescriptor, buildTrackingFormDefaults } from '$lib/providers';
+	import ErrorBanner from '$lib/components/ErrorBanner.svelte';
+	import Button from '$lib/components/Button.svelte';
 	import type { TrackingConfig } from '$lib/types';
 
 	/** Grid-select item source lookup — maps descriptor string name to actual function. */
@@ -83,10 +85,9 @@
 </script>
 
 <PageHeader title={t('trackingConfig.title')} description={t('trackingConfig.description')}>
-	<button onclick={() => { showForm ? (showForm = false, editing = null) : openNew(); }}
-		class="px-3 py-1.5 bg-[var(--color-primary)] text-[var(--color-primary-foreground)] rounded-md text-sm font-medium hover:opacity-90">
+	<Button size="sm" onclick={() => { showForm ? (showForm = false, editing = null) : openNew(); }}>
 		{showForm ? t('common.cancel') : t('trackingConfig.newConfig')}
-	</button>
+	</Button>
 </PageHeader>
 
 {#if !loaded}<Loading />{:else}
@@ -94,7 +95,7 @@
 {#if showForm}
 <div in:slide={{ duration: 200 }}>
 <Card class="mb-6">
-	{#if error}<div class="bg-[var(--color-error-bg)] text-[var(--color-error-fg)] text-sm rounded-md p-3 mb-4">{error}</div>{/if}
+	{#if error}<ErrorBanner message={error} />{/if}
 	<form onsubmit={save} class="space-y-5">
 		<div>
 			<label for="tc-name" class="block text-sm font-medium mb-1">{t('trackingConfig.name')}</label>
@@ -194,9 +195,9 @@
 		</Card>
 		{/if}
 
-		<button type="submit" class="px-4 py-2 bg-[var(--color-primary)] text-[var(--color-primary-foreground)] rounded-md text-sm font-medium hover:opacity-90">
+		<Button type="submit">
 			{editing ? t('common.save') : t('common.create')}
-		</button>
+		</Button>
 	</form>
 </Card>
 </div>