refactor(assets): add LlmProvider and Language enums with strum

- Add LlmProvider enum with strum for string parsing and IconNamed impl
- Add Language enum for programming languages with IconNamed impl
- Add IntoIcon trait for easy Icon conversion
- Implement IntoElement for direct use in .child()
- Reorganize icons into ui/, llm_provider/, language/ directories
- Add rewrite_path to Assets for icon path compatibility
- Update chat_sidebar and settings_view to use new enums

Author: AprilNEA

Cargo.lock

@@ -78,6 +78,9 @@ chrono = "0.4"
 # i18n
 rust-i18n = "3"
 
+# Enum utilities
+strum = { version = "0.26", features = ["derive"] }
+
 # Database
 postgresql_embedded = { version = "0.20", features = ["bundled"], optional = true }
 sea-orm = { version = "1.1", features = ["runtime-tokio-rustls", "macros"] }