Fix table rendering in markdown with improved glamour styling

[Copilot]

The table rendering in assistant messages was poorly formatted with misaligned columns and overlapping content. As shown in the issue screenshot, tables appeared with broken alignment, content wrapping incorrectly within cells, and poor visual hierarchy.

Problem:
The glamour StyleTable configuration was minimal, lacking proper margin, indentation, and color settings. This resulted in:

• Poor column alignment
• Content overlapping with table borders
• Inconsistent spacing
• Difficult to read table structure

Solution:
Enhanced the table styling configuration in internal/tui/styles/markdown.go by adding:

Table: ansi.StyleTable{
    StyleBlock: ansi.StyleBlock{
        StylePrimitive: ansi.StylePrimitive{
            BlockPrefix: "\n",
            BlockSuffix: "\n",
            Color:       stringPtr(adaptiveColorToString(t.MarkdownText())),
        },
        Margin: uintPtr(defaultMargin),      // Added proper spacing
        Indent: uintPtr(1),                  // Added indentation for hierarchy
    },
    CenterSeparator: stringPtr("┼"),
    ColumnSeparator: stringPtr("│"),
    RowSeparator:    stringPtr("─"),
},

Results:

• Tables are now properly indented with consistent spacing
• Column content is better organized and aligned
• Text flows more naturally within cells
• Improved visual hierarchy and readability
• Consistent theming with the rest of the application

Testing:
Added comprehensive tests to validate the table rendering improvements and ensure no regressions in existing functionality.

Fixes #18.

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