feat(web): show the Inspector version in the lower-right corner (#1639)

Display the single-source version (root package.json) in the web UI's
lower-right corner, in grey — matching the CLI and TUI. The browser can't read
the filesystem, so the version flows backend → browser via GET /api/config:

- Add `version?: string` to InitialConfigPayload; the web backend
  (webServerConfigToInitialPayload) reads it once via the shared
  readInspectorVersion() and tags it onto the payload.
- Add a `useInspectorVersion` hook mirroring useSandboxUrl/useServerListWritable.
- Add a presentational `VersionBadge` element rendered fixed lower-right, grey,
  non-interactive; styling lives in a `versionBadge` Text theme variant (no
  inline styles, --inspector-* tokens).
- Wire it into App.tsx.

Verified live: /api/config serves `version: "2.0.0"` and the badge renders
`v2.0.0` fixed at the lower-right in grey. Tests: useInspectorVersion (full
branch coverage), VersionBadge (unit + Storybook), web-server-config payload.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BrihGWcrM9JGRyu41nzZYw

clients/web/src/components/elements/VersionBadge/VersionBadge.test.tsx

@@ -0,0 +1,22 @@
+import { describe, it, expect } from "vitest";
+import { renderWithMantine, screen } from "../../../test/renderWithMantine";
+import { VersionBadge } from "./VersionBadge";
+
+describe("VersionBadge", () => {
+  it("renders the version with a `v` prefix and an accessible label", () => {
+    renderWithMantine(<VersionBadge version="2.0.0" />);
+    const badge = screen.getByText("v2.0.0");
+    expect(badge).toBeInTheDocument();
+    expect(badge).toHaveAttribute("aria-label", "Inspector version 2.0.0");
+  });
+
+  it("renders nothing when the version is undefined", () => {
+    renderWithMantine(<VersionBadge version={undefined} />);
+    expect(screen.queryByText(/^v/)).toBeNull();
+  });
+
+  it("renders nothing when the version is an empty string", () => {
+    renderWithMantine(<VersionBadge version="" />);
+    expect(screen.queryByText(/^v/)).toBeNull();
+  });
+});