Improve bootstrap static theme: visible search feedback, loading state, data-i18n-alt

[marevol]

Summary

Functional review fixes for the bundled bootstrap reference static theme. Three gaps were found where the theme consumes the /api/v2/* API correctly but the front-end did not give the user complete feedback:

1. Visible search-failure feedback. Network, server (5xx) and auth failures during a search were written only to the screen-reader-only #results-meta sink, so a sighted user saw stale/blank results with no indication the search had failed. These errors are now surfaced in the visible #search-error banner (which is already role="alert"), falling back to #results-meta only when a theme has no visible banner. The invalid_request/400 path is unchanged.

2. Search loading indicator. The cache and chat views had a loading state; the main search did not, so a slow query left the previous results on screen with no progress hint. A #search-loading spinner is now shown while a /search request is in flight and hidden when it settles. The hide is guarded so a superseded request never clears the spinner belonging to a newer one. A new labels.search_loading key was added to all 16 message bundles.

3. data-i18n-alt support. Both logos carried a data-i18n-alt attribute, but i18n.js applyDom() only processed data-i18n / -placeholder / -aria-label, so the intended localized alt text was never applied. applyDom() now handles data-i18n-alt. The home logo referenced a key (labels.index_title) that did not exist in any bundle; it has been repointed to the existing page.heading key (the header brand already uses the existing labels.header_brand_name).

Changes

• assets/i18n.js — applyDom() now localizes alt via data-i18n-alt.
• assets/search.js — visible error feedback for network/server/auth failures; showSearchLoading() helper + supersede-safe loading toggle in runSearch().
• index.html — #search-loading indicator in the results view; home logo data-i18n-alt repointed to page.heading.
• i18n/messages.*.json (16) — new labels.search_loading key.
• BundledBootstrapThemeTest — 7 regression tests covering the three fixes.

Testing

mvn test -Dtest='org.codelibs.fess.theme.*Test,org.codelibs.fess.i18n.LabelMessageThemeParityTest' — all green (BundledBootstrapThemeTest 120, LabelMessageThemeParityTest 5+2, ThemeManifestTest 11, StaticThemeResponderTest 34, StaticThemeInstallerTest 23, ThemeRegistryTest 7). i18n key parity preserved across all 16 bundles.