feat(instances): choose the instance, and keep accounts on several

A new install asks first which instance to keep the person's studysets
on, as Mastodon's apps do: an address, checked to answer as an instance
of Halkyon Learn, or the build's own (learn.halkyon.eu) in one tap.
Nothing is made anywhere before that, not even an anonymous account. An
install from before goes on with the build's instance without asking.

As Moshidon does, the device keeps an account on each instance it was
used with, each signed in with its own folder and session, one in use at
a time. Settings › Switch account lists them: another one switches at
once, anonymous accounts included; Add account chooses another instance;
removing one not in use deletes what the device holds of it. Signing out
ends only the account in use.

The build's own instance keeps its data where installs kept it before
(`people/`, Serverpod's own session key), so they keep everything; other
instances get folders and keys of their own. What the device shares
across instances (its settings, sounds, the Pomodoro timer's place in the
system) is made once and outlives every switch.

The F-Droid listing drops TetheredNet: the app is no longer tied to the
project's instance.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

README.md

@@ -113,7 +113,8 @@ Releases are published on [code.halkyon.app](https://code.halkyon.app/project-ha
 with the APK and the snap, and the APK also in the project's F-Droid repository
 (`https://halkyon.app/fdroid/repo?fingerprint=E8082788756F6B7E682AA22FCEAD24B53E2A8B0912AD66D65E8C2D4F07BB94B0`,
 added in the F-Droid app as a repository). The Snap Store (`halkyon-learn`) follows.
-Their builds connect to learn.halkyon.eu, the project's instance.
+Their builds offer learn.halkyon.eu, the project's instance, at first start; people
+may choose another there, and keep accounts on several.
 
 - **Checking a download:** `SHA256SUMS` lists the checksum of every file, signed with the
   maintainer's GPG key (`SHA256SUMS.asc`). The APK is signed with the app's key, whose