feat(usage): enhance dashboard with auto-refresh control and robust formatting

- Add configurable auto-refresh interval toggle (off/5s/10s/30s/60s) to usage dashboard
- Extract shared format utilities (fmtUsd, fmtInt, parseFiniteNumber, getLocaleFromLanguage)
- Refactor request log time filtering to rolling vs fixed mode with validation
- Use stable serializable query keys instead of filter objects
- Handle NaN/Infinity safely in number formatting across all usage components
- Use RFC 3339 date format in backend trend data

src-tauri/src/services/usage_stats.rs

@@ -272,7 +272,7 @@ impl Database {
                 .single()
                 .unwrap_or_else(Local::now);
 
-            let date = bucket_start.format("%Y-%m-%dT%H:%M:%S").to_string();
+            let date = bucket_start.to_rfc3339();
 
             if let Some(mut stat) = map.remove(&i) {
                 stat.date = date;