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/targets/TargetForm.svelte

@@ -9,6 +9,7 @@
 	import MultiEntitySelect from '$lib/components/MultiEntitySelect.svelte';
 	import type { EntityItem } from '$lib/components/EntitySelect.svelte';
 	import type { GridItem } from '$lib/components/IconGridSelect.svelte';
+	import Button from '$lib/components/Button.svelte';
 
 	interface Props {
 		form: {
@@ -184,7 +185,7 @@
 			<label class="flex items-center gap-2 text-sm"><input type="checkbox" bind:checked={form.ai_captions} /> {t('targets.aiCaptions')}<Hint text={t('hints.aiCaptions')} /></label>
 		{/if}
 
-		<button type="submit" disabled={submitting} class="px-4 py-2 bg-[var(--color-primary)] text-[var(--color-primary-foreground)] rounded-md text-sm font-medium hover:opacity-90 disabled:opacity-50">{submitting ? t('common.loading') : (editing ? t('common.save') : t('targets.create'))}</button>
+		<Button type="submit" disabled={submitting}>{submitting ? t('common.loading') : (editing ? t('common.save') : t('targets.create'))}</Button>
 	</form>
 </Card>
 </div>