Guide shows where to point, explains each song fact, and works with no song open

[thcp]

Three gaps in help mode, found by using it.

What changes

• A glow when Guide turns on. Every control Guide can explain glows gold for about a second and a half, then fades, so a new user sees where help exists and the screen stays calm afterwards. Large areas such as the waveforms are left out.
• Each song fact explains itself. Key (and what its small percentage means), BPM, LUFS, duration, dynamic range and tempo stability each have their own box, and every presence card names its instrument: "How loud the Drums part is next to the loudest instrument in the song". The wording matches how the numbers are computed (runner.py presence, analyze.py dynamic range and stability).
• It works with no song open. The studio turns Export Mix, the song details button and the analysis cards off for the pointer while it is empty, so help mode never heard it over them. In help mode they receive the pointer again; Guide still takes every click, so nothing is pressed by accident.

All new texts are in the ten languages.

Testing

• tests/e2e/help-mode.spec.mjs: 3 new tests (the glow appears and clears, each card explains itself, an empty studio still explains Export Mix, song details, Key and Global Vocals). 9 pass.
• Full Playwright suite on next-release plus this and A We Recommend card can link to only one place, and Adrianna Claro is missing #761: 464 passed. One unrelated lyrics test flaked once and passed 24 of 24 on rerun.
• Automated test run with real separation on an isolated server: 71 of 71 assertions, including these three checks.

Closes #760