fix: SFTP default to Local pane + fixed border height

- Default active pane changed from Remote to Local on SFTP init
- renderPane now takes maxH parameter for fixed border height
- paneStyle.Height(maxH) prevents border from shrinking/growing on scroll
- Stacked mode: maxH = t.height - 6; Side-by-side: t.height - 5

CHANGELOG.md

@@ -33,6 +33,10 @@
 - Added pane indicator bar: `[Local]  Remote` with active pane highlighted
 - Side-by-side mode (≥50 cols) unchanged — both panes remain visible
 - Tab key now visually switches between panes on narrow terminals
+- Default active pane: Local (was Remote)
+- Border height fix: pane uses fixed `Height(maxH)` — border no longer shrinks/grows when scrolling
+- `renderPane` now takes `maxH` parameter to control content area height
+- Stacked: `maxH = t.height - 6`; Side-by-side: `maxH = t.height - 5`
 - Breakpoints: compact (<60), medium (60–100), wide (≥100)
 
 ## v1.0.0 (2025-01-30)