feat: add server-side image comparison shortcode support

- Add {{< img-compare >}} and [img-compare] shortcode syntax
- Implement server-side shortcode processing in Rust
- Create dedicated shortcode module for extensibility
- Fix image comparison slider display issues
- Remove caption display for cleaner UI
- Update to version 0.2.6

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ailog"
-version = "0.2.5"
+version = "0.2.6"
 edition = "2021"
 authors = ["syui"]
 description = "A static blog generator with AI features"