perf: SFTP directory listing cache

- Added dirCache map to sftpPane for caching directory listings
- Cache hit: instant navigation (no disk/network I/O)
- Cache miss: read from disk/SFTP, store in cache per path
- R key clears current directory cache and forces fresh read
- Significant speedup when navigating back to previously visited folders

CHANGELOG.md

@@ -39,6 +39,13 @@
 - Stacked: `maxH = t.height - 6`; Side-by-side: `maxH = t.height - 5`
 - Breakpoints: compact (<60), medium (60–100), wide (≥100)
 
+### SFTP Performance Fix
+- Added directory listing cache (`dirCache map[string][]os.FileInfo`) to each pane
+- Cache hit: instant navigation (no disk/network call)
+- Cache miss: read from disk/SFTP, store in cache
+- `R` key now clears cache for current directory and forces re-read
+- Cache is per-path: navigating to a previously visited folder is instant
+
 ## v1.0.0 (2025-01-30)
 
 ### Added