refactor(prompt): extract file path logic and implement auto-import on first launch (#214)

- Extract prompt file path logic to dedicated prompt_files module
- Refactor PromptService to use centralized path resolution
- Implement auto-import for existing prompt files on first startup
- Add comprehensive unit tests for auto-import functionality

src-tauri/Cargo.toml

@@ -57,3 +57,7 @@ lto = "thin"
 opt-level = "s"
 panic = "abort"
 strip = "symbols"
+
+[dev-dependencies]
+serial_test = "3"
+tempfile = "3"