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/src/models/connection.rs

@@ -0,0 +1,41 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ConnectionConfig {
+    pub id: String,
+    pub name: String,
+    pub host: String,
+    pub port: u16,
+    pub user: String,
+    pub password: String,
+    pub database: String,
+    pub ssl_mode: Option<String>,
+    pub color: Option<String>,
+}
+
+impl ConnectionConfig {
+    pub fn connection_url(&self) -> String {
+        let ssl = self.ssl_mode.as_deref().unwrap_or("prefer");
+        format!(
+            "postgres://{}:{}@{}:{}/{}?sslmode={}",
+            urlencoded(&self.user),
+            urlencoded(&self.password),
+            self.host,
+            self.port,
+            urlencoded(&self.database),
+            ssl
+        )
+    }
+}
+
+fn urlencoded(s: &str) -> String {
+    s.chars()
+        .map(|c| match c {
+            ':' | '/' | '?' | '#' | '[' | ']' | '@' | '!' | '$' | '&' | '\'' | '(' | ')'
+            | '*' | '+' | ',' | ';' | '=' | '%' | ' ' => {
+                format!("%{:02X}", c as u8)
+            }
+            _ => c.to_string(),
+        })
+        .collect()
+}