Add height option to fuzzy finder and improve list height calculation

[ditahkk]

This PR adds comprehensive dimension control and a robust layout system to the fuzzy finder, providing greater control over its appearance and fixing critical feature coordination issues.

Key Changes:

• Customizable Border

  • Introduced WithBorder() and WithBorderChars() options to enable and style a window border.
  • The rendering logic was updated to draw the border and adjust UI element coordinates.
  • The default border style is ╭╮╰╯─│.

• Configurable Height and Width

  • Added WithHeight() and WithWidth() options to set fixed dimensions.
  • The fuzzy finder now respects these dimensions, falling back to the terminal size if not specified.
  • Fixed: When WithHeight() is used, the finder now positions at the bottom of the terminal (like fzf), not centered.

• Layout System Architecture (Major Refactor)

  • Created centralized Layout abstraction system with rect type for rectangular areas.
  • Implemented computeLayout() function as single source of truth for all UI component positions.
  • Refactored all drawing functions (_draw(), _drawPreview(), _drawBorder()) to use shared layout coordinates.
  • Fixed critical coordination issues:
    • Preview panel now respects height limits and border offsets correctly
    • All features (border, preview, height/width constraints) now coordinate properly
    • Eliminated independent coordinate calculations that caused conflicts
  • Added comprehensive test suite for layout validation and edge cases.

• Code Quality Improvements

  • Removed unused listWidth() function.
  • Updated golden test files for new correct rendering behavior.
  • All code properly formatted with zero compiler warnings.
  • Added extensive unit and integration tests for layout system.

• Dependency Management & Fixes

  • Added go-isatty and pflag dependencies for the example application.
  • Fixed unused parameter bug and removed unused variables.

• Example and Documentation

  • Updated the example CLI to demonstrate custom border characters.
  • Added a "Development" section to README.md with build and test instructions.

Technical Details:

Before: Features calculated positions independently, causing conflicts:

• Preview panel ignored height limits (used f.term.Size() directly)
• Preview panel ignored border offsets (drew at absolute coordinates)
• Height-constrained boxes were centered instead of bottom-aligned
• 100+ lines of redundant offset calculations scattered across drawing functions

After: Clean architecture with single layout calculation:

• All features use shared Layout struct with pre-calculated coordinates
• Preview panel properly respects all constraints
• Bottom-aligned positioning when height is constrained
• Easy to add new features (footer, multi-column, etc.)
• Fully testable layout logic in isolation

Testing:

All tests pass (1.6s runtime):

• ✅ Unit tests for rect helper methods
• ✅ Layout validation tests for edge cases
• ✅ Integration tests for all option combinations
• ✅ Updated golden files for correct rendering behavior
• ✅ Zero warnings or errors

Breaking Changes:

None. While the internal architecture was significantly refactored, the public API remains unchanged and all existing tests pass.