feat(agent): pluggable database drivers via JDBC agent subprocess

Migrate domestic database support (DaMeng, KingBase, Vastbase, GoldenDB) from
built-in ODBC/protocol drivers to standalone Java agent processes communicating
via stdin/stdout JSON-RPC 2.0. Agents are distributed as fat JARs with embedded
JDBC drivers, downloaded on-demand from GitHub Releases with proxy fallback.

- Add AgentDriverClient for subprocess lifecycle + JSON-RPC communication
- Add AgentManager for JRE/JAR install state management
- Add Tauri commands for driver install/uninstall/list
- Add DriverManager UI panel in settings dialog
- Extend DatabaseType with Kingbase, Vastbase, Goldendb variants
- Migrate Dameng from ODBC (dm_driver.rs) to JDBC agent
- Delete dm_driver.rs (343 lines)
- Add GitHub proxy fallback for downloads (update.hwdns.net, gh-proxy.org)

Agent code lives at https://github.com/t8y2/dbx-agents

src/lib/databaseCapabilities.ts

@@ -7,6 +7,8 @@ export const SCHEMA_AWARE_TYPES = new Set<DatabaseType>([
   "redshift",
   "dameng",
   "gaussdb",
+  "kingbase",
+  "vastbase",
   "jdbc",
 ]);
 
@@ -21,6 +23,9 @@ export const DIAGRAM_SUPPORTED_TYPES = new Set<DatabaseType>([
   "redshift",
   "dameng",
   "gaussdb",
+  "kingbase",
+  "vastbase",
+  "goldendb",
 ]);
 
 export const DATABASE_SEARCH_SUPPORTED_TYPES = new Set<DatabaseType>([
@@ -34,6 +39,9 @@ export const DATABASE_SEARCH_SUPPORTED_TYPES = new Set<DatabaseType>([
   "clickhouse",
   "dameng",
   "gaussdb",
+  "kingbase",
+  "vastbase",
+  "goldendb",
 ]);
 
 export const TABLE_IMPORT_SUPPORTED_TYPES = new Set<DatabaseType>([
@@ -49,6 +57,9 @@ export const TABLE_IMPORT_SUPPORTED_TYPES = new Set<DatabaseType>([
   "redshift",
   "dameng",
   "gaussdb",
+  "kingbase",
+  "vastbase",
+  "goldendb",
 ]);
 
 export const TABLE_STRUCTURE_SUPPORTED_TYPES = new Set<DatabaseType>(["mysql", "postgres", "sqlite", "sqlserver"]);
@@ -85,6 +96,8 @@ export const TREE_SCHEMA_TYPES = new Set<DatabaseType>(["postgres", "redshift",
 
 export const PG_LIKE_STRUCTURE_TYPES = new Set<DatabaseType>(["postgres", "redshift", "gaussdb"]);
 
+export const AGENT_DRIVER_TYPES = new Set<DatabaseType>(["dameng", "kingbase", "vastbase", "goldendb"]);
+
 export const TRANSFER_SQL_TYPES = new Set<DatabaseType>([
   "mysql",
   "postgres",