feat(templates): complete Tera migration, remove maud

Fully migrate from compile-time maud templates to runtime Tera:
- Rewrote main.rs to use TemplateEngine and discover_sections()
- Replaced hardcoded blog/projects with generic section loop
- Added Clone derive to Frontmatter and Content
- Fixed section_type dispatch via Section struct
- Deleted src/templates.rs, removed maud dependency

Users can now add sections without code changes.

Cargo.toml

@@ -7,7 +7,6 @@ version     = "0.1.0"
 
 [dependencies]
 gray_matter    = "0.2"
-maud           = "0.26"
 pulldown-cmark = "0.12"
 thiserror      = "2"
 walkdir        = "2"