feat: add ER diagram and enhance TableStructure with FK details, triggers, comments

- Add interactive ER diagram with ReactFlow + dagre auto-layout, accessible
  via right-click context menu on schema nodes in the sidebar
- Enhance TableStructure: column comments, FK referenced table/columns,
  ON UPDATE/DELETE rules, new Triggers tab
- Backend: rewrite get_table_constraints using pg_constraint for proper
  composite FK support, add get_table_triggers and get_schema_erd commands

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src-tauri/src/models/schema.rs

@@ -18,6 +18,7 @@ pub struct ColumnInfo {
     pub ordinal_position: i32,
     pub character_maximum_length: Option<i32>,
     pub is_primary_key: bool,
+    pub comment: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -34,6 +35,11 @@ pub struct ConstraintInfo {
     pub name: String,
     pub constraint_type: String,
     pub columns: Vec<String>,
+    pub referenced_schema: Option<String>,
+    pub referenced_table: Option<String>,
+    pub referenced_columns: Option<Vec<String>>,
+    pub update_rule: Option<String>,
+    pub delete_rule: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -43,3 +49,48 @@ pub struct IndexInfo {
     pub is_unique: bool,
     pub is_primary: bool,
 }
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct TriggerInfo {
+    pub name: String,
+    pub event: String,
+    pub timing: String,
+    pub orientation: String,
+    pub function_name: String,
+    pub is_enabled: bool,
+    pub definition: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ErdColumn {
+    pub name: String,
+    pub data_type: String,
+    pub is_nullable: bool,
+    pub is_primary_key: bool,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ErdTable {
+    pub schema: String,
+    pub name: String,
+    pub columns: Vec<ErdColumn>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ErdRelationship {
+    pub constraint_name: String,
+    pub source_schema: String,
+    pub source_table: String,
+    pub source_columns: Vec<String>,
+    pub target_schema: String,
+    pub target_table: String,
+    pub target_columns: Vec<String>,
+    pub update_rule: String,
+    pub delete_rule: String,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ErdData {
+    pub tables: Vec<ErdTable>,
+    pub relationships: Vec<ErdRelationship>,
+}