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/users/+page.svelte

@@ -12,6 +12,8 @@
 	import EmptyState from '$lib/components/EmptyState.svelte';
 	import IconButton from '$lib/components/IconButton.svelte';
 	import { snackSuccess, snackError } from '$lib/stores/snackbar.svelte';
+	import ErrorBanner from '$lib/components/ErrorBanner.svelte';
+	import Button from '$lib/components/Button.svelte';
 	import type { User } from '$lib/types';
 
 	const auth = getAuth();
@@ -67,17 +69,16 @@
 </script>
 
 <PageHeader title={t('users.title')} description={t('users.description')}>
-	<button onclick={() => showForm = !showForm}
-		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}>
 		{showForm ? t('users.cancel') : t('users.addUser')}
-	</button>
+	</Button>
 </PageHeader>
 
 {#if !loaded}<Loading />{:else}
 
 {#if showForm}
 	<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={create} class="space-y-3">
 			<div>
 				<label for="usr-name" class="block text-sm font-medium mb-1">{t('users.username')}</label>
@@ -94,7 +95,7 @@
 					<option value="admin">{t('users.roleAdmin')}</option>
 				</select>
 			</div>
-			<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">{t('users.create')}</button>
+			<Button type="submit">{t('users.create')}</Button>
 		</form>
 	</Card>
 {/if}
@@ -110,7 +111,7 @@
 			<div class="flex items-center justify-between">
 				<div>
 					<p class="font-medium">{user.username}</p>
-					<p class="text-sm text-[var(--color-muted-foreground)]">{user.role} · {t('users.joined')} {new Date(user.created_at).toLocaleDateString()}</p>
+					<p class="text-sm text-[var(--color-muted-foreground)]">{user.role === 'admin' ? t('users.roleAdmin') : t('users.roleUser')} · {t('users.joined')} {new Date(user.created_at).toLocaleDateString()}</p>
 				</div>
 				<div class="flex items-center gap-1">
 					{#if user.id !== auth.user?.id}
@@ -137,9 +138,9 @@
 		{#if resetMsg}
 			<p class="text-sm {resetSuccess ? 'text-[var(--color-success-fg)]' : 'text-[var(--color-error-fg)]'}">{resetMsg}</p>
 		{/if}
-		<button type="submit" class="w-full py-2 bg-[var(--color-primary)] text-[var(--color-primary-foreground)] rounded-md text-sm font-medium hover:opacity-90">
+		<Button type="submit" class="w-full">
 			{t('common.save')}
-		</button>
+		</Button>
 	</form>
 </Modal>