Add Linux font fallbacks for Mermaid SVG text rendering

[1jehuang]

crates/jcode-tui-mermaid/src/mermaid_content.rs font stack lacks Linux-native fallbacks (DejaVu Sans, Liberation Sans), which can produce invisible/missing Mermaid labels with resvg on Linux.

Credit: originally proposed in PR #108. This repo no longer accepts external PRs, so the problem is tracked here instead.

[1jehuang]

Triage: confirmed in crates/jcode-tui-mermaid/src/mermaid_content.rs:267 (terminal_theme). The font stack is "Inter, ui-sans-serif, system-ui, -apple-system, Segoe UI, sans-serif" with no Linux-native fallbacks (DejaVu Sans, Liberation Sans). resvg resolves fonts via a fontdb loaded with load_system_fonts() (lib.rs:422-426), and the primary family is picked as the first comma-entry (mermaid_svg.rs:primary_font_family, defaulting to "Inter"). On Linux systems without Inter/Segoe UI installed, label text can fail to resolve a glyph face and render missing/invisible.

Fix path: add DejaVu Sans / Liberation Sans (and Noto Sans) to the family list so a commonly-installed Linux face is matched. Low-risk, localized; autonomous: likely.

[1jehuang]

Fixed in b96dfa47 (pending release). Built and tests pass on Linux. See commit message for the specific change for this issue.

[1jehuang]

Released in v0.32.0. Run jcode update to get it.