fix(theme): let a renamed copy of the bootstrap theme load its own assets

[marevol]

Why

The page design editor is gone, so the way to change the look of the search pages is to copy the bundled bootstrap static theme. Following the bundled README (copy the directory, rename it, change theme.yml#name, upload it as a ZIP), the copy's CSS, logo and message edits never appeared: its index.html hardcoded themes/bootstrap/ for the stylesheet, both logos and app.js, and app.js loads the i18n and help bundles relative to itself, so the copy kept running the bootstrap files.

Relative URLs cannot fix this alone. Fess serves index.html at /, /search, /help, the error paths and so on, and inserts <base href="{context path}/">, so a relative URL resolves against the web application root at every depth (/search/x too), never against the theme's directory.

What changes

• StaticThemeResponder replaces {{themePath}} in the entry file with themes/<theme name>, the path StaticThemeFilter serves the theme's assets under. It applies to both the normal and the error-page responses, since both read the entry through readEntryBytes(). A theme without the placeholder is served unchanged, so existing themes that name their own directory keep working.
• The bundled index.html refers to its stylesheet, logos and app.js as {{themePath}}/assets/….
• The README's Context path section documents the placeholder, and its Customising steps are rewritten: copy and rename, set name (and bump version), edit, ZIP with theme.yml at the top level, upload, then set the copy as the default or bind it to a virtual host.
• Tests: StaticThemeResponderTest covers the expansion on serveIndex (under a context path) and serveErrorPage, and the unchanged pass-through. BundledBootstrapThemeTest requires index.html not to name themes/bootstrap and every {{themePath}} reference to exist.

The theme version stays 1.3.0: it was set for 15.9 and has not been released yet.

Verification

• mvn test -Dtest=BundledBootstrapThemeTest,StaticThemeResponderTest,StaticThemeFilterTest,ErrorPageServletTest: 230 tests passed.
• Headless Chromium against a distribution built with this change, OpenSearch 3.8. The bundled theme was copied as mytheme following the README, with one rule added to its styles.css (body { background-color: rgb(253, 246, 227) !important; }), uploaded on the admin Theme page and made the default. Before (main) → after, on /, /search?q=… and a 404 at a two-segment path:

Check	Before	After
Active theme (/api/v2/ui/config)	mytheme	mytheme
Stylesheet / module script	themes/bootstrap/assets/styles.css, themes/bootstrap/assets/app.js	themes/mytheme/assets/styles.css, themes/mytheme/assets/app.js
Logos	themes/bootstrap/assets/logo*.png	themes/mytheme/assets/logo*.png
i18n bundle requested	/themes/bootstrap/i18n/messages.ja.json	/themes/mytheme/i18n/messages.ja.json
Computed body background	rgb(255, 255, 255) (edit ignored)	rgb(253, 246, 227)