Fix front-matter formatting in talk pages and clean navigation whitespace

[jychen74]

Motivation

• Ensure talk markdown files have valid YAML front matter delimiters and placement so the static site generator parses metadata correctly.
• Remove stray trailing spaces and normalize commented lines in the navigation file to avoid spurious diffs and formatting issues.
• Keep page content (authors, abstracts, images) outside the YAML front matter for correct rendering.

Description

• Replaced and normalized front matter delimiters and placement in multiple talk files by moving the --- markers to properly wrap metadata in _talks/2024-asv-pabv-diagnostic-talk.md, _talks/2025-NCB-獸醫診斷研究的跨界旅程-talk.md, _talks/2025-asv-comparative-pabv-talk.md, and _talks/2025-tamu-integrated-transcriptomic-analysis-talk.md so metadata is parsed correctly and body content follows the front matter.
• Restored the expected three-dash YAML front matter syntax where files had incorrect or misplaced dashes and ensured closing --- is present before main content.
• Cleaned up trailing whitespace and normalized commented entries and spacing in _data/navigation.yml to make header links consistent and reduce formatting noise.

Testing

• No automated tests were run for these content and formatting-only changes.

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Codex Task