feat: add Tauri v2 Rust backend with PostgreSQL support

Add Rust backend: AppState with connection pool management,
TuskError handling, ConnectionConfig model, and all commands
for connections, schema browsing, query execution, data CRUD,
and CSV/JSON export via sqlx.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -0,0 +1,32 @@
+[package]
+name = "tusk"
+version = "0.1.0"
+description = "PostgreSQL Database Management GUI"
+authors = ["you"]
+license = ""
+repository = ""
+edition = "2021"
+rust-version = "1.77.2"
+
+[lib]
+name = "tusk_lib"
+crate-type = ["staticlib", "cdylib", "rlib"]
+
+[build-dependencies]
+tauri-build = { version = "2", features = [] }
+
+[dependencies]
+tauri = { version = "2", features = [] }
+tauri-plugin-shell = "2"
+tauri-plugin-dialog = "2"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+tokio = { version = "1", features = ["full"] }
+sqlx = { version = "0.8", features = ["runtime-tokio-rustls", "postgres", "json", "chrono", "uuid", "bigdecimal"] }
+chrono = { version = "0.4", features = ["serde"] }
+uuid = { version = "1", features = ["serde"] }
+thiserror = "2"
+csv = "1"
+log = "0.4"
+hex = "0.4"
+bigdecimal = { version = "0.4", features = ["serde"] }