fix: keep the site up when a Vite build is missing

[ogorzalka]

Fixes Pollora/pollora#79.

Problem

When a theme's build is missing (npm absent, build failed, public/build deleted), every request answered 500, wp-login.php and wp-admin included: Vite manifest not found at public/build/theme/default/manifest.json.

Thrown while WordPress boots: init → block registration → BlockRegistrar::registerScriptHandle() → ViteManager::getAssetUrls() → the getAssetUrls Vite macro → Vite::manifest(). The layout's Asset::url() reads the same manifest through ViteManager::asset().

Fix

• ViteManager::isMissingBuild(): neither a running dev server nor a manifest. Then getAssetUrls() returns ['js' => [], 'css' => []] and asset() an empty string, instead of throwing. Blocks register without scripts, theme assets are left out.
• MissingBuilds (singleton) records each missing build: one Log::warning per container per request, with the manifest path.
• MissingBuildNotice on admin_notices (users with manage_options): one line per manifest with the containers using it, npm install && npm run build (ddev npm … under DDEV) and php artisan pollora:doctor, whose builds check names the folder.

Tests

• Unbuilt container: no URLs, asset() → '', no exception.
• Recorded once with its manifest path, a single warning logged across several lookups.
• Nothing recorded for a built container or one served by the dev server.
• Notice: names the containers and manifest once, and both commands; silent when every build is there.

Checked on a real site

On pollora-debugbar, build hidden (public/build/theme/default renamed):

• before: /, a page, wp-login.php, wp-admin → 500 Vite manifest not found
• after: / → 200, wp-login.php → 200 (login form), wp-admin → login redirect; log: Pollora: the "theme" assets are not built, no Vite manifest at …; admin_notices for an admin prints "Pollora: assets not built — theme.blocks, theme — no manifest at public/build/theme/default/manifest.json".
• build restored: notice gone, pages 200.

Changelog: [Unreleased] → ### Fixed.