feat: Implement hierarchical sidebar with projects and tags

Add complete sidebar implementation with:
- ProjectTree model using arena allocator for hierarchical projects
- FilterState model for global filter state management
- Sidebar view with independent scrollable sections
- Project tree with expand/collapse functionality
- Tag selection with toggle support
- Observable filter state shared across components

src/main.rs

@@ -2,7 +2,9 @@ use crate::app::App;
 
 mod app;
 mod components;
+mod models;
 mod theme;
+mod view;
 
 fn main() {
     App::run();