feat: add configurable layout direction

[julianStreibel]

Summary

• Add layout_direction config option to switch between horizontal (side-by-side) and vertical (stacked) panel layout
• Uses int mapping: 0 = horizontal (default), 1 = vertical
• Zero-value default ensures horizontal layout when config option is not set

Changes

• internal/config/config.go — New LayoutDirection int type with LayoutHorizontal/LayoutVertical constants, added to Config struct with load/save support
• internal/ui/model.go — Conditional layout switching in View() based on config value
• config.example.toml — Document the new option

Test plan

• go build ./... passes
• Launch with default config — horizontal layout unchanged
• Set layout_direction = 1 — panels stack vertically
• Omit layout_direction from config — defaults to horizontal

[julianStreibel]

@rose-m what do you think? This saves some space on multi split views.

[rose-m]

Sounds good 👍