refactor(fxt): extract FILTER_ALL constant for gRPC "fetch all" empty string filters

Replace three occurrences of String::new() used as "fetch all" filter
values in gRPC request construction with a named FILTER_ALL constant,
making the intent explicit.

The api_gateway monitoring handler was reviewed but its Prometheus URL
appears only once in tests, so no constant was extracted (single-use).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bin/fxt/src/tui/data_fetcher.rs

@@ -29,6 +29,9 @@ use super::state::{
     RiskAlertData, RiskData, ServiceData, SystemResources, TrainingSessionData,
 };
 
+/// Empty filter string means "fetch all" in gRPC requests.
+const FILTER_ALL: &str = "";
+
 // ---------------------------------------------------------------------------
 // StateUpdate – one variant per data source
 // ---------------------------------------------------------------------------
@@ -143,7 +146,7 @@ impl DataFetcher {
             let mut backoff = Backoff::new();
             loop {
                 let req = Request::new(monitoring::StreamTrainingMetricsRequest {
-                    model_filter: String::new(),
+                    model_filter: FILTER_ALL.into(),
                     interval_seconds: 3,
                 });
                 match mon.stream_training_metrics(req).await {
@@ -420,7 +423,7 @@ impl DataFetcher {
             let mut backoff = Backoff::new();
             loop {
                 let req = Request::new(trading::StreamPortfolioSummaryRequest {
-                    account_id: String::new(),
+                    account_id: FILTER_ALL.into(),
                     interval_seconds: 3,
                 });
                 match trd.stream_portfolio_summary(req).await {
@@ -716,7 +719,7 @@ impl DataFetcher {
             let mut backoff = Backoff::new();
             loop {
                 let req = Request::new(ml::StreamModelStatusRequest {
-                    model_name: String::new(), // all models
+                    model_name: FILTER_ALL.into(), // all models
                     interval_seconds: 5,
                 });
                 match ml_client.stream_model_status(req).await {