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/theme/Text.ts

@@ -37,6 +37,23 @@ export const ThemeText = Text.extend({
         },
       };
     }
+    // A small, unobtrusive build-version label pinned to the lower-right corner
+    // of the screen (#1639): grey, non-interactive (clicks pass through), and
+    // out of the tab/selection flow so it never interferes with the UI beneath.
+    if (props.variant === "versionBadge") {
+      return {
+        root: {
+          position: "fixed",
+          bottom: "0.35rem",
+          right: "0.6rem",
+          zIndex: 100,
+          color: "var(--inspector-text-secondary)",
+          fontSize: "var(--mantine-font-size-xs)",
+          pointerEvents: "none",
+          userSelect: "none",
+        },
+      };
+    }
     return { root: {} };
   },
 });