feat(ui): Improve stack view preview highlighting and item-wise navigation

[mhiro2]

Summary

This PR improves Stack View readability and navigation.
It adds Tree-sitter-based syntax highlighting to preview lines while reducing base preview highlight priority to avoid visual noise.
It also updates cursor movement to be stack-item-oriented and prevents the cursor from landing on header lines.

Changes

• Apply Tree-sitter highlights to preview lines in Stack View
• Lower base preview highlight priority so Tree-sitter highlights remain readable
• Dynamically align the preview marker (│) with each stack item prefix width
• Change j/k to move by stack item (skip preview/header lines)
• Change gg/G to jump to first/last stack item
• Prevent the cursor from landing on header lines such as Stack: N
• Update stack view key behavior docs in README.md and doc/peekstack.txt
• Expand tests/stack_view_spec.lua coverage for:
  • Tree-sitter apply/skip/fallback/truncation clamping
  • Dynamic preview alignment
  • Item-wise cursor movement and header-line guard