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/bots/EmailBotTab.svelte

@@ -131,10 +131,9 @@
 				<input type="checkbox" bind:checked={emailForm.smtp_use_tls} />
 				{t('emailBot.useTls')}
 			</label>
-			<button type="submit" disabled={emailSubmitting}
-				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">
+			<Button type="submit" disabled={emailSubmitting}>
 				{emailSubmitting ? t('common.loading') : (editingEmail ? t('common.save') : t('emailBot.addBot'))}
-			</button>
+			</Button>
 		</form>
 	</Card>
 {/if}
@@ -157,7 +156,7 @@
 							<span class="text-xs text-[var(--color-muted-foreground)] font-mono">{bot.email}</span>
 							<span class="text-xs px-1.5 py-0.5 rounded bg-[var(--color-muted)] text-[var(--color-muted-foreground)]">{bot.smtp_host}:{bot.smtp_port}</span>
 							{#if bot.smtp_use_tls}
-								<span class="text-xs px-1.5 py-0.5 rounded bg-emerald-500/10 text-emerald-500">TLS</span>
+								<span class="text-xs px-1.5 py-0.5 rounded bg-[var(--color-success-bg)] text-[var(--color-success-fg)]">TLS</span>
 							{/if}
 						</div>
 					</div>