refactor: restructure project directories to follow FHS

- Move themes/ → data/themes/ (installable to /usr/share/owlry/themes/)
- Move resources/ → src/resources/ (embedded into binary)
- Move config.example.toml → data/ (installable to /usr/share/doc/owlry/)
- Update include_str! paths in app.rs

New structure follows Filesystem Hierarchy Standard:
- data/         → files installed to /usr/share/
- src/resources → embedded resources compiled into binary

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app.rs

@@ -82,7 +82,7 @@ impl OwlryApp {
 
         // 1. Load base structural CSS (always applied)
         let base_provider = CssProvider::new();
-        base_provider.load_from_string(include_str!("../resources/base.css"));
+        base_provider.load_from_string(include_str!("resources/base.css"));
         gtk4::style_context_add_provider_for_display(
             &display,
             &base_provider,
@@ -95,7 +95,7 @@ impl OwlryApp {
             let theme_provider = CssProvider::new();
             match theme_name.as_str() {
                 "owl" => {
-                    theme_provider.load_from_string(include_str!("../resources/owl-theme.css"));
+                    theme_provider.load_from_string(include_str!("resources/owl-theme.css"));
                     debug!("Loaded built-in owl theme");
                 }
                 _ => {