[Yannicked]

Stacked on #40. One commit each:

• Drop the always-on SEND_RICH_TEXT switch (Slack refusing blocks already falls back to text), a stale probe note, and a lint allow without a reason.
• lock_whole where poisoned locks were unwrapped by hand.
• retry::backoff for the huddle check and poll waits. Behaviour change: Trouter reconnects backed off never (5 s forever); now 5 s doubling to 5 min, reset after a connection that lasted a minute.
• One plain_text instead of two; SetupForm::app() for the three places building AppCredentials (its secrets are Secrets, so it derives Debug).
• Shared setup for the three Teams call entry points; InMeeting derives Default.
• One picker row for the switcher and the command palette; the people lookup paths shared with the probe.
• The pretend Chime servers shared between the loopback tests.

Looked at and left: sharing the decoder plumbing of gallery.rs and screen.rs (~25 lines for medium risk).

+308 / −378 lines.