feat: play the menu ambience on the title from the first gesture

[r4mbo7]

Closes #181
Closes #179

Cause: the title was muted on purpose in soundOf, and no AudioContext was created before Jouer, Jouer ensemble or the online lobby (checked on the deployed site: zero contexts, even after a key press).

• The title plays the menu ambience, which already fades into the set at launch. The sound switch and the hidden tab still silence it.
• The first click, key or pad button opens the audio.
• The master chain opens muted when the sound is off, so no blip before the first ramp.
• Tests: soundOf unit test, and e2e/title-music.spec.ts (context running and output gain at 1 after a first key; gain 0 with the sound off). Both failed before the fix.

Title ambience, 20 s: https://github.com/r4mbo7/Ozoboom/blob/pr-assets/issue-181/ecran-titre-20s.mp3 (WAV next to it).