feat: implement lazygit-compatible scrollbars in list views

[felipecm-br]

Description

Implements exact lazygit-compatible vertical scrollbar rendering logic across all scrollable list views and diffs.

Details

• Ported calcScrollbar, calcScrollbarHeight, and calcRealScrollbarStartEnd from lazygit (pkg/gocui/scrollbar.go and gui.go).
• Renders the vertical scroll indicator using ▐ (Right Half Block) on the right panel border with matching theme border styling.
• Integrates into list views (render_list_with_range and render_list_with_range_raw) and side-by-side / unified diff views.
• Comprehensive unit tests matching lazygit's scrollbar_test.go verifying thumb height, scroll range, and boundary conditions.

Validation

• cargo test --bin lazygitrs (all 378 tests passed)
• cargo fmt --check

[felipecm-br]

This is how the scroll looks like :)