refactor: remove standalone proxies, add Traefik provider with Docker labels

Standalone proxy removal:
- Delete store, API handlers, proxy manager, health monitor, validator, hints
- Delete frontend pages (proxies list, create, edit) and components (ProxyCard, ProxyForm, ProxyFilter, ProxyGroup, ValidationChecklist)
- Remove proxy routes from router, nav items, dashboard references
- Clean up SystemHealthCard to remove proxy section

Traefik provider:
- Add TraefikProvider implementing proxy.Provider via Docker labels
- ContainerLabels() returns traefik.enable, router rule, entrypoints, service port, TLS cert resolver, docker network
- ConfigureRoute() returns router name (labels handle routing at container creation)
- DeleteRoute() is no-op (container removal auto-deregisters)
- Ping() checks Traefik API health (optional)
- Wire ContainerLabels into deployer (executeDeploy + blueGreenDeploy)
- Add Traefik settings: entrypoint, cert_resolver, network, api_url
- Add traefik option to proxy provider selector in settings UI
- Show conditional Traefik config fields
- Add i18n keys (EN + RU)

internal/store/store.go

@@ -104,6 +104,11 @@ func (s *Store) runMigrations() error {
 		`ALTER TABLE instances ADD COLUMN proxy_route_id TEXT NOT NULL DEFAULT ''`,
 		// Add proxy_provider to settings (2026-04-04). Default to npm for backward compat.
 		`ALTER TABLE settings ADD COLUMN proxy_provider TEXT NOT NULL DEFAULT 'npm'`,
+		// Add Traefik provider settings (2026-04-04).
+		`ALTER TABLE settings ADD COLUMN traefik_entrypoint TEXT NOT NULL DEFAULT 'websecure'`,
+		`ALTER TABLE settings ADD COLUMN traefik_cert_resolver TEXT NOT NULL DEFAULT 'letsencrypt'`,
+		`ALTER TABLE settings ADD COLUMN traefik_network TEXT NOT NULL DEFAULT ''`,
+		`ALTER TABLE settings ADD COLUMN traefik_api_url TEXT NOT NULL DEFAULT ''`,
 	}
 
 	for _, m := range migrations {
@@ -202,8 +207,12 @@ CREATE TABLE IF NOT EXISTS settings (
 	webhook_secret      TEXT NOT NULL DEFAULT '',
 	polling_interval    TEXT NOT NULL DEFAULT '5m',
 	base_volume_path   TEXT NOT NULL DEFAULT '',
-	ssl_certificate_id INTEGER NOT NULL DEFAULT 0,
-	updated_at          TEXT NOT NULL DEFAULT (datetime('now'))
+	ssl_certificate_id     INTEGER NOT NULL DEFAULT 0,
+	traefik_entrypoint     TEXT NOT NULL DEFAULT 'websecure',
+	traefik_cert_resolver  TEXT NOT NULL DEFAULT 'letsencrypt',
+	traefik_network        TEXT NOT NULL DEFAULT '',
+	traefik_api_url        TEXT NOT NULL DEFAULT '',
+	updated_at             TEXT NOT NULL DEFAULT (datetime('now'))
 );
 
 CREATE TABLE IF NOT EXISTS instances (