Fix recording genre artwork under public base paths

[hotzenplotz5]

Visible failure

Recordings 2 rendered the Genre artwork elements, but the cover areas were black when VDR-Suite was served below the public base path /vdr-suite. Ordinary folders such as Ghibli continued to show the normal folder icon.

Confirmed root cause

web/frontend/recordings2-folder-artwork.js assigned root-relative CSS image URLs directly for both the Genre sprite and the individual Genre SVGs. The module therefore depended on global CSS property/prototype adapters to rewrite style.backgroundImage, which is not a sufficiently reliable module contract.

What changed

• add a small publicPath() helper that explicitly calls VdrSuitePublicUrl.resolvePath() when the public URL runtime is available;
• resolve the shared Genre sprite through that helper;
• resolve every individual Genre SVG through that helper;
• retain the direct-daemon root-relative path as the tested fallback when the resolver is unavailable;
• leave Genre mapping, sprite positions, single-recording leaf handling and ordinary-folder behavior unchanged.

Regression coverage

The new focused Recordings-2 frontend test proves:

• empty public base path keeps the Sprite and individual SVG paths root-relative;
• /vdr-suite prefixes both paths exactly once;
• /vdr-suite/vdr-suite/... is never produced;
• the module works without any available CSSStyleDeclaration prototype patch;
• the established Genre slugs and Sprite positions remain unchanged;
• an unmapped folder such as Ghibli still returns no Genre artwork and therefore keeps the normal folder presentation;
• the existing exported Recordings-2 folder-artwork runtime API remains intact;
• the resolver-unavailable direct-daemon fallback remains covered.

The test is registered in the existing test-recordings2-runtime target; no new Make target was invented.

Validation

Executed in the agent's isolated validation workspace against the exact changed file contents:

• node --check web/frontend/recordings2-folder-artwork.js
• node --check web/frontend/tests/test_recordings2_folder_artwork_public_url.js
• node web/frontend/tests/test_recordings2_folder_artwork_public_url.js
• git diff --check, including a separate whitespace check for the new untracked test fixture

The complete existing Recordings-2 and frontend contract targets are left to the repository's GitHub Actions checkout, because the canonical yaVDR checkout is not mounted in the agent environment.

Real-system evidence

Before this repository fix, the equivalent bounded change was applied directly to /usr/share/vdr-suite/web/frontend/recordings2-folder-artwork.js on the real system. node --check succeeded there and the user confirmed: Die Cover sind wieder da. This is positive runtime evidence for the approach, but the direct /usr/share edit is not treated as repository source of truth or as proof that this branch has already been installed.

Scope and safety

This PR changes only the Recordings-2 frontend module, its focused frontend test and the existing Recordings-2 test registration. It changes no daemon code, database, recording cache, artwork cache, series-artwork provider, TVmaze logic, TMDB logic or PR #133 state.