fix: dynamically read version from manifest in About modal

[Copilot]

The About modal had 1.2 hardcoded, diverging from the splash screen which already fetches version from manifest.webmanifest.

Changes

• Added module-level appVersion variable (fallback '1.2') to share version across functions
• loadManifestVersion() now writes fetched version to appVersion before updating versionEl
• openAppModal() interpolates appVersion instead of the literal string

// Before
: '<h2>EasyTune</h2><p>... Wersja PWA offline 1.2.</p>';

// After
: `<h2>EasyTune</h2><p>... Wersja PWA offline ${appVersion}.</p>`;