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/lib/components/MultiEntitySelect.svelte

@@ -105,7 +105,7 @@
 	{/if}
 	<button type="button" class="mes-trigger" onclick={openPalette}>
 		<MdiIcon name="mdiPlus" size={14} />
-		<span>{(values || []).length === 0 ? placeholder : `${(values || []).length} selected`}</span>
+		<span>{(values || []).length === 0 ? placeholder : t('common.nSelected').replace('{0}', String((values || []).length))}</span>
 		<span class="mes-trigger-arrow"><MdiIcon name="mdiChevronDown" size={14} /></span>
 	</button>
 </div>
@@ -120,7 +120,7 @@
 			<input
 				bind:this={inputEl}
 				bind:value={query}
-				placeholder="Search..."
+				placeholder={t('common.search')}
 				class="mes-input"
 				type="text"
 				autocomplete="off"