fix(tools): resolve external MySQL/PG client tools correctly (#44)

Addresses #44 (closed after the v0.8.3 release builds are green).

Three independent defects in one shared tool-resolution path: bundled tools were
looked up at the wrong path (so bundled fallback never worked, for PostgreSQL
too), macOS GUI apps inherit launchd's minimal PATH, and only mariadb-dump/mariadb
were probed — never mysqldump/mysql, which is what brew install mysql-client
installs. Both engines now resolve through one headless, cached, timed resolver,
plus a non-blocking client/server version warning.

Also fixes passing MySQL-only CLI options to the bundled MariaDB client, and
carries the tool-page UI polish and the requested #40 icon revert.

desktop/src/stores/backupStore.test.ts

@@ -0,0 +1,36 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+const listeners = vi.hoisted(
+  () => ({} as Record<string, (e: { payload: unknown }) => void>),
+);
+vi.mock("@tauri-apps/api/event", () => ({
+  listen: vi.fn(async (name: string, cb: (e: { payload: unknown }) => void) => {
+    listeners[name] = cb;
+    return () => {};
+  }),
+}));
+
+import { useBackupStore } from "./backupStore";
+import { useNotificationStore } from "./notificationStore";
+
+describe("backupStore advisory warning", () => {
+  beforeEach(() => {
+    useNotificationStore.setState({ notifications: [] });
+    useBackupStore.setState({ jobs: [], activeJobId: null, progress: 0 });
+  });
+
+  it("surfaces a non-fatal warning as an info notification", async () => {
+    await useBackupStore.getState().initListener();
+    listeners["backup-progress"]({
+      payload: {
+        job_id: "j1",
+        status: "running",
+        progress: 0,
+        warning: "Client/server version mismatch: the resolved dump tool is old",
+      },
+    });
+    const notes = useNotificationStore.getState().notifications;
+    expect(notes.some((n) => n.message.includes("version mismatch"))).toBe(true);
+    expect(notes.some((n) => n.type === "info")).toBe(true);
+  });
+});