client: show build version + browser diagnostics for bug triage

Fixes #68.

- Bar switcher and stats page get a low-contrast "wutzcalc <git-sha>"
  footer at the bottom (VersionFooter component, shared).
- Admin panel gets a fuller Diagnose section: version, user agent,
  screen resolution + device pixel ratio, viewport size, browser
  language.

Version is the short git commit sha, baked in at build time via
vite.config.ts define (falls back to "dev" if .git is unavailable at
build time, e.g. a tarball/CI-artifact deploy) - matches how deploys
actually run (pnpm build from a git checkout, per
deploy/wutzcalc.service), but stays defensive rather than failing the
build.

pnpm --filter client typecheck and build both clean.

client/src/tablet/BarPicker.tsx

@@ -1,6 +1,7 @@
 import { useState } from 'preact/hooks';
 import type { Bar } from '@wutzcalc/shared';
 import { getTheme, setTheme } from '../api';
+import { VersionFooter } from '../VersionFooter';
 
 interface Props {
   bars: Bar[] | null;
@@ -35,6 +36,7 @@ export function BarPicker({ bars, onPick }: Props) {
       <button class="theme-toggle" onClick={toggleTheme}>
         {theme === 'dark' ? '☀ Heller Modus' : '🌙 Dunkler Modus'}
       </button>
+      <VersionFooter />
     </div>
   );
 }