No console windows on Windows: the last two places

Added on top of the contribution, one commit, 2 files.

1. The voice installer and voice samples run hidden too

Installing the narration voice from the settings and playing a voice sample there run uv and Python. They used Node's process API, which passes its own window setting, so the contribution's hidden-by-default did not reach them: each step opened a console window for seconds to minutes. They now ask for no window.

- spawn(cmd, args, { stdio: ['ignore', 'pipe', 'pipe'], …env })
+ spawn(cmd, args, { stdio: ['ignore', 'pipe', 'pipe'], windowsHide: true, …env })

2. The design doc describes the behaviour as it is now

The new paragraph in docs/design/running-obeya.md now describes the behaviour in the present tense and also names the voice installer. The decision (a default for every program rather than an option at each call) moved into the chapter's list of decisions:

+ **Programs run without a window on Windows by default, not by an option at each call**
+ (2026-11-01): one call forgotten brings the flashing windows back. A program meant to be seen asks
+ for its window.