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/components/config/DriverManager.vue

@@ -0,0 +1,94 @@
+<script setup lang="ts">
+import { ref, onMounted } from "vue";
+import { invoke } from "@tauri-apps/api/core";
+import { Button } from "@/components/ui/button";
+
+interface AgentDriverInfo {
+  db_type: string;
+  label: string;
+  version: string;
+  size: number;
+  installed: boolean;
+  installed_version: string | null;
+  update_available: boolean;
+}
+
+const drivers = ref<AgentDriverInfo[]>([]);
+const jreInstalled = ref(false);
+const installing = ref<string | null>(null);
+
+onMounted(async () => {
+  await refresh();
+});
+
+async function refresh() {
+  jreInstalled.value = await invoke<boolean>("check_jre_installed");
+  drivers.value = await invoke<AgentDriverInfo[]>("list_installed_agents");
+}
+
+async function installDriver(dbType: string) {
+  installing.value = dbType;
+  try {
+    await invoke("install_agent", { dbType });
+    await refresh();
+  } catch (e: any) {
+    alert(e);
+  } finally {
+    installing.value = null;
+  }
+}
+
+async function uninstallDriver(dbType: string) {
+  try {
+    await invoke("uninstall_agent", { dbType });
+    await refresh();
+  } catch (e: any) {
+    alert(e);
+  }
+}
+
+function formatSize(bytes: number): string {
+  return `${(bytes / 1024 / 1024).toFixed(1)} MB`;
+}
+</script>
+
+<template>
+  <div class="space-y-4 p-4">
+    <h3 class="text-lg font-medium">驱动管理</h3>
+
+    <div class="text-sm text-muted-foreground">
+      JRE: {{ jreInstalled ? "✓ 已安装" : "未安装" }}
+      <span v-if="!jreInstalled" class="ml-2 text-xs">(首次安装驱动时自动下载)</span>
+    </div>
+
+    <div class="space-y-2">
+      <div
+        v-for="driver in drivers"
+        :key="driver.db_type"
+        class="flex items-center justify-between rounded-md border p-3"
+      >
+        <div>
+          <span class="font-medium">{{ driver.label }}</span>
+          <span v-if="driver.installed" class="ml-2 text-xs text-muted-foreground">
+            v{{ driver.installed_version }}
+          </span>
+          <span class="ml-2 text-xs text-muted-foreground">{{ formatSize(driver.size) }}</span>
+        </div>
+        <div>
+          <Button
+            v-if="!driver.installed"
+            size="sm"
+            :disabled="installing !== null"
+            @click="installDriver(driver.db_type)"
+          >
+            {{ installing === driver.db_type ? "安装中..." : "安装" }}
+          </Button>
+          <div v-else class="flex items-center gap-2">
+            <span class="text-sm text-green-600">✓ 已安装</span>
+            <Button size="sm" variant="ghost" @click="uninstallDriver(driver.db_type)">卸载</Button>
+          </div>
+        </div>
+      </div>
+    </div>
+  </div>
+</template>