fix(android): a chosen server starts fresh, without the last one behind it

Found on a real device: after Change server, Back did not offer the leave
prompt but reversed into the previous server's pages, leaving the app showing
one instance while it was set to another. The address form is reached through
Back, so switching twice was enough to get there.

The behaviour predates the recent-servers list, but that list is what turns
switching servers into an everyday action rather than a once-a-year one.

Only a deliberate choice of server clears the history - typed in, picked from
the list, or restored at startup. Following a notification into a conversation
still goes back the way it came.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/android-app.md

@@ -168,6 +168,10 @@ actually answered — a mistyped address never joins it. If you run a single
 instance you will simply see it listed there on its own, and nothing about the
 form changes.
 
+Connecting to a server this way starts it fresh: **Back** brings up the leave
+prompt again rather than reversing into the pages of the server you just left.
+Otherwise the app would end up showing one instance while it is set to another.
+
 ### Servers that ask for a password
 
 If your instance sits behind a reverse proxy configured to require a login