feat(observability): phases 4-7 - complete frontend UI (big bang)

Add all frontend pages for observability & proxy management:
- Proxy Viewer: /proxies with grouped view, filtering, health indicators
- Proxy Creation: form with live validation, diagnostic hints, edit/delete
- Stale Containers: /containers/stale with dashboard widget, cleanup actions
- Event Log: /events with filters, pagination, real-time SSE streaming
- Navigation: proxies and events links in sidebar
- i18n: full EN/RU translations for all new features
- Settings: stale threshold configuration

web/src/routes/settings/+page.svelte

@@ -20,6 +20,7 @@
 	let pollingInterval = $state('');
 	let baseVolumePath = $state('');
 	let notificationUrl = $state('');
+	let staleThresholdDays = $state('7');
 
 	let sslCertificateId = $state(0);
 	let sslCertName = $state('');
@@ -79,6 +80,7 @@
 			baseVolumePath = settings.base_volume_path ?? '';
 			sslCertificateId = settings.ssl_certificate_id ?? 0;
 			notificationUrl = settings.notification_url ?? '';
+			staleThresholdDays = String(settings.stale_threshold_days ?? 7);
 		} catch (err) {
 			toasts.error(err instanceof Error ? err.message : $t('settingsGeneral.loadFailed'));
 		} finally {
@@ -101,7 +103,8 @@
 				domain: domain.trim(), server_ip: serverIp.trim(), network: network.trim(),
 				subdomain_pattern: subdomainPattern.trim(), polling_interval: pollingInterval.trim(),
 				base_volume_path: baseVolumePath.trim(), notification_url: notificationUrl.trim(),
-				ssl_certificate_id: sslCertificateId
+				ssl_certificate_id: sslCertificateId,
+				stale_threshold_days: Math.max(1, parseInt(staleThresholdDays, 10) || 7)
 			});
 			toasts.success($t('settingsGeneral.saved'));
 		} catch (err) {
@@ -242,6 +245,21 @@
 				</div>
 			</div>
 
+			<!-- Stale Detection -->
+			<div class="mt-6 border-t border-[var(--border-primary)] pt-4">
+				<h3 class="mb-3 text-sm font-semibold text-[var(--text-primary)]">{$t('stale.title')}</h3>
+				<div class="max-w-xs">
+					<FormField
+						label={$t('settings.staleThreshold')}
+						name="staleThresholdDays"
+						type="number"
+						bind:value={staleThresholdDays}
+						placeholder="7"
+						helpText={$t('settings.staleThresholdHelp')}
+					/>
+				</div>
+			</div>
+
 			<div class="mt-6">
 				<button onclick={handleSave} disabled={saving} class="inline-flex items-center gap-2 rounded-lg bg-[var(--color-brand-600)] px-4 py-2.5 text-sm font-medium text-white shadow-sm transition-all duration-150 hover:bg-[var(--color-brand-700)] disabled:opacity-50 active:animate-press">
 					{#if saving}<IconLoader size={16} />{/if}