feat: add connection colors, query history, SQL autocomplete, and EXPLAIN visualizer

Add four developer/QA features:
- Connection color coding: color picker in dialog, colored indicators across toolbar, tabs, status bar, and connection selectors
- Query history: Rust backend with JSON file storage (500 entry cap), sidebar panel with search/clear, auto-recording from workspace
- Schema-aware SQL autocomplete: backend fetches column metadata, CodeMirror receives schema namespace with public tables unprefixed
- EXPLAIN ANALYZE visualizer: recursive tree view with cost-colored bars, expand/collapse nodes, buffers info, Results/Explain tab toggle

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

src-tauri/src/commands/schema.rs

@@ -2,6 +2,7 @@ use crate::error::{TuskError, TuskResult};
 use crate::models::schema::{ColumnInfo, ConstraintInfo, IndexInfo, SchemaObject};
 use crate::state::AppState;
 use sqlx::Row;
+use std::collections::HashMap;
 use tauri::State;
 
 #[tauri::command]
@@ -340,3 +341,40 @@ pub async fn get_table_indexes(
         })
         .collect())
 }
+
+#[tauri::command]
+pub async fn get_completion_schema(
+    state: State<'_, AppState>,
+    connection_id: String,
+) -> TuskResult<HashMap<String, HashMap<String, Vec<String>>>> {
+    let pools = state.pools.read().await;
+    let pool = pools
+        .get(&connection_id)
+        .ok_or(TuskError::NotConnected(connection_id))?;
+
+    let rows = sqlx::query(
+        "SELECT table_schema, table_name, column_name \
+         FROM information_schema.columns \
+         WHERE table_schema NOT IN ('pg_catalog', 'information_schema', 'pg_toast') \
+         ORDER BY table_schema, table_name, ordinal_position",
+    )
+    .fetch_all(pool)
+    .await
+    .map_err(TuskError::Database)?;
+
+    let mut result: HashMap<String, HashMap<String, Vec<String>>> = HashMap::new();
+    for row in &rows {
+        let schema: String = row.get(0);
+        let table: String = row.get(1);
+        let column: String = row.get(2);
+
+        result
+            .entry(schema)
+            .or_default()
+            .entry(table)
+            .or_default()
+            .push(column);
+    }
+
+    Ok(result)
+}