feat: dual-pane SFTP browser (local ↔ remote)

Phase B — Tamagosh-inspired dual-pane file browser:
- Left pane: local filesystem (starts at /Users/swanadiva)
- Right pane: remote filesystem via SFTP
- Tab key switches active pane (green border = active)
- Local CWD indicator per pane
- File listing with dir/ file size display
- Filter mode with / key
- Copy files: C key copies from active pane to opposite pane
- Delete (D), mkdir (N on remote), Refresh (R)
- Esc to close
- File size formatting (B/KiB/MiB/GiB)
- Progress message during transfers
- StylePaneActive (green border) / StylePaneInactive (muted border)

pkg/tui/styles.go

@@ -45,3 +45,9 @@ func TabBarWidth(totalWidth int) int {
 	}
 	return totalWidth - 2
 }
+
+// Pane styles for dual-pane layout
+var (
+	StylePaneActive   = lipgloss.NewStyle().Border(lipgloss.RoundedBorder()).BorderForeground(gbGreen).Padding(0, 1)
+	StylePaneInactive = lipgloss.NewStyle().Border(lipgloss.RoundedBorder()).BorderForeground(gbBorder).Padding(0, 1)
+)