Fix flag SVG 404 by resolving asset bundle base URL alias

[luke-]

Problem

Bundled team flag SVGs 404 on HumHub develop, e.g.
…/kickoff/c/@web/assets/<hash>/flags/1f1f2-1f1fd.svg — note the literal @web in the path.

Cause

views/competition/_team_badge.php built the flag URL by concatenating the published asset bundle's baseUrl straight into the <img src>:

$baseUrl = Assets::register($this)->baseUrl;
$flagUrl = "{$baseUrl}/flags/{$fileStem}.svg";

A bundle's baseUrl is not guaranteed to be a final URL. HumHub develop's new filesystem-backed AssetManager sets the base URL from the assets mount config without resolving aliases ($this->baseUrl = Yii::$app->fs->getAssetsMountConfig()->getBaseUrl()), so it is the literal path alias @web/assets/<hash>. HumHub resolves that alias only when it emits <link>/<script> tags — Html::cssFile()/Html::jsFile() run the URL through Url::to(). The hand-built <img src> skipped that step, so the literal @web/... ended up in the markup and the browser resolved it relative to the current page (…/kickoff/c/@web/assets/…) → 404. Registered CSS was unaffected for exactly this reason. On stable it happened to work because the older asset manager resolved baseUrl at init.

Note: AssetManager::getAssetUrl($bundle, 'flags/x.svg') would not fix this — it returns $bundle->baseUrl . '/' . $asset and isn't overridden on develop, so it yields the same unresolved @web/....

Fix

Resolve the flag URL through Url::to(), mirroring HumHub's own asset-URL handling (Html::cssFile()→Url::to()). Extracted into Assets::flagUrl(). This is a no-op for the already-resolved /assets/<hash> form on stable, so it is fully backwards-compatible, and also handles absolute base URLs (console/cron render) and the app-at-root case (@web = '').

Added AssetsTest covering the unresolved-alias and already-resolved cases.

CI didn't catch this because no test renders the badge.