feat(l1): Save per channel, and Keep messages Off deletes the log at the next start

Channel options > Save keeps a busy channel's new messages off the flash, so
it leaves the room to the others; Keep messages stays the master switch.
Off now only stops the saving, so a value stepped past costs nothing and the
wait for the delete comes at a start. Delete history stays immediate.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/solo/settings.md

@@ -39,8 +39,9 @@ A few notes:
 - **Keep messages** (Wio Tracker L1) saves messages on the flash, across a
   power-off and past what fits in memory (see
   [How much is kept](./messages.md#how-much-is-kept)): **Off**, 500, 1000 or
-  2500 in total. When the limit is reached the oldest go, in batches. Setting
-  it to Off deletes what was saved.
+  2500 in total. When the limit is reached the oldest go, in batches. **Off**
+  stops saving at once and deletes what was saved at the next start, so turn it
+  back on before then to keep it. **Delete history** deletes right away.
 - **Delete after** (Wio Tracker L1) removes saved messages older than 7, 30 or
   90 days, or **Never**. They go in batches too, so some stay a little longer,
   and nothing goes while the clock isn't set.