fix(tui): add plain font style fallback

[kupendrav]

Fixes #330

Testing:

• cargo check --package jcode-tui --lib --message-format short
• cargo test --package jcode-tui --lib tui::ui::font_style_tests::strip_font_variant_modifiers_preserves_ascii_symbols -- --exact
• git diff --check

This does not look like message data corruption. The regression test keeps the exact ASCII text browser continue Done Built in the buffer, then removes only terminal font style modifiers. The symbols remain unchanged.

That points to a terminal/font rendering problem: some styled glyph variants are missing or rendered as boxes. This patch adds an opt-in fallback:

JCODE_TUI_PLAIN_FONT_STYLE=1

When enabled, the final TUI frame strips bold/dim/italic/reverse modifiers before Ratatui writes the buffer, avoiding bad font variants while preserving text, colors, layout, and transcript data.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds an opt-in “plain font style” mode for the TUI by stripping certain font-variant modifiers (bold/dim/italic/reversed) from the rendered buffer when an environment variable is set.

Changes:

• Introduces JCODE_TUI_PLAIN_FONT_STYLE environment flag parsing.
• Post-processes the frame buffer to remove selected ratatui Modifiers when enabled.
• Adds a unit test ensuring symbol contents are preserved while modifiers are stripped.

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