chore: migrate non-blog content to YAML frontmatter

Convert all content files to use YAML (---) instead of TOML (+++)
for consistent parsing:

- content/_index.md (homepage)
- content/about.md, content/collab.md
- content/blog/_index.md
- content/projects/_index.md + 14 project cards

Enables unified gray_matter YAML parser across all content.

content/projects/_index.md

@@ -1,5 +1,3 @@
-+++
-title = "Projects"
-sort_by = "weight"
-template = "cards.html"
-+++
+---
+title: Projects
+---