fix(widgets): table row selection shows full background color

[vch9]

Summary

Fixes table row selection highlighting to display full background color across the entire row when selection_mode = Row.

Problem

When navigating through a table with row selection enabled, only the vertical border characters (│) between columns displayed the selection background color. The cell contents remained unstyled, making it difficult to see which row was selected.

Root cause: Each border character was styled with themed_border, which adds ANSI codes ending with \033[0m (reset all formatting). When themed_selection wrapped the entire assembled row, these reset codes cleared the selection background for all content between borders.

Solution

• Pass ~is_selected flag to assemble_columns() function
• When is_selected = true, render border characters as plain text (no themed_border)
• Border characters now inherit the selection background applied to the full row
• Applied to both wrapped and non-wrapped rendering paths

Testing

You can test the fix with the table demo:

```bash
cd path/to/miaou
dune exec example/demos/table/main.exe
```

Use Up/Down arrows to navigate between rows. You should see the entire selected row with a colored background, not just the vertical separators.

• All existing tests pass (dune runtest)
• Verified compilation with dune build)
• Manual testing with table demo shows full row highlighting

Version Bump

• Version bumped from 0.4.0 to 0.4.1 (bug fix release)
• CHANGELOG.md updated with new [0.4.1] section
• All opam files regenerated with new version

Checklist

• Code compiles without warnings
• All tests pass
• CHANGELOG.md updated
• Version bumped appropriately
• Commit message follows conventional format
• Change is backward compatible (no API changes)