feat: Add a system info screen, with the details of each speaker

[abusch]

Adds a fullscreen system info screen, opened with i, for troubleshooting.

It shows the household ID and the number of speakers and groups, then for each speaker:

• its group, whether it coordinates it, and the group's playback state (playing, paused, idle or buffering)
• its volume, IP address and ID
• its software and API versions, and whether it is registered (in red when it isn't)
• its capabilities, with the Sonos API's names, wrapped to fit
• each of its devices: model name and code, serial number, hardware and software versions, Sonos generation and colour, plus the ID and address of devices other than the main one (e.g. a sub)

↑↓/jk scroll, PGUP/PGDN scroll by a page, HOME/END jump to either end, and ESC, i or q close it. It doesn't scroll past the last line, and shows a scrollbar when it doesn't all fit.

To get there, HouseholdState now keeps the household ID and a shared copy of the topology, and Playback keeps the playback state. The labels use a new label theme colour (cyan, or the theme's secondary accent), to stand out from their values. serde_json is a new dev-dependency, to build a speaker in the tests.

Tried read-only against a real household of five speakers, including a home theatre.

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