feat: show build SHA + timestamp in a small footer for deploy verification

Adds a BuildStamp component pinned to the bottom-right of every page
(desktop only, dim monospace text) so it's obvious at a glance which
build is currently running.

The git short SHA and an ISO build timestamp are injected at build time
via Vite's `define`, falling back to 'unknown' if git is not available.
ESLint config declares the two compile-time constants as readonly
globals so no per-file disable comments are needed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

eslint.config.js

@@ -14,7 +14,11 @@ export default defineConfig([
       reactRefresh.configs.vite,
     ],
     languageOptions: {
-      globals: globals.browser,
+      globals: {
+        ...globals.browser,
+        __BUILD_SHA__: 'readonly',
+        __BUILD_TIME__: 'readonly',
+      },
       parserOptions: { ecmaFeatures: { jsx: true } },
     },
     rules: {