Fix startup crash from invalid Devanagari font

[imnakul]

Summary

• Replace the invalid HTML file bundled as Noto Sans Devanagari with the verified Google Fonts TrueType asset and include its SIL Open Font License and source hashes.
• Exercise the production egui font setup in a headless frame test covering proportional, monospace, and Devanagari fallback text.
• Record bounded panic source metadata and eframe startup errors in %LOCALAPPDATA%\Venu\startup.log, without storing panic payload text.

Verification

• Headless production-font test passes with the replacement asset and fails with the original invalid asset at egui's InvalidFont panic.
• Windows-target cargo check against the application source passes using an external manifest.
• cargo fmt --check, git diff --check, and font provenance hash validation pass.

This remains a v0.4.3 repair patch; it does not change the version or published release.