Fix table text copy not matching selection

[yunus25jmi1]

Description

Fixes issue #2167 where text copied from tables did not match the visual selection.

Root Cause

When selecting text in tables, the copied content did not match the visual selection because border characters (│, ─, etc.) were not properly accounted for when mapping visual column positions to text content.

The previous implementation used a simple offset calculation that subtracted the total border width from column positions, but this didn't account for the actual positions of border characters within the line.

Changes

pkg/tui/components/messages/clipboard.go

• Added visualToRune mapping to track which runes correspond to which visual columns
• Implemented findClosestRuneIndex() helper function to map visual column positions to rune indices
• Updated extractSelectedText() to properly map visual selection positions to text content

How It Works

1. When extracting selected text, the code now builds a map of visual column positions to rune indices in the stripped text
2. For each rune in the original line, it tracks whether it's a border character (skipped) or content (kept)
3. The visual column position is calculated by summing the display width of each rune
4. When extracting the selection, the code maps the visual start/end columns to the corresponding rune indices in the stripped text

Testing

• ✅ Code compiles successfully
• ✅ Existing tests pass
• ✅ Manual testing required for table selection in TUI

Related Issue

Fixes #2167