feat(l1): messages kept on flash across a power-off, and "Powered off" on e-ink

Settings > Messages > Keep messages (Off, 500, 1000, 2500 in all; off by
default; HIST_FLASH, ui-core/HistoryLog.h). Two append-only logs on the QSPI
flash, channels and DMs, restored into the RAM rings at the start. A batch is
written a minute after the first unsaved change and at shutdown, as the L1's
power switch gives no warning. Each segment has a directory of its own: a
LittleFS 1.x commit erases one of a directory's two fixed blocks every time.

E-ink panels keep their last frame, so a shutdown now leaves "Powered off" on
it and puts the controller into deep sleep.

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

docs/solo/developer/build-flags.md

@@ -117,4 +117,5 @@ values above that are set from the phone app or the CLI.
 | `ADVERT_NAME='"name"'` | Sets the default node name baked into a fresh device, instead of the hex of the first 4 bytes of its public key. Note the doubled quoting — it's a C string literal passed through a build flag. |
 | `MAP_VECTOR` | Wio Tracker L2: turns on the unfinished vector map (regions from `tools/maps/osm_vector.py`). Without it the vector map stays off whatever was saved. |
 | `HIST_ARCHIVE` | Wio Tracker L2: message history also kept on the SD card (`ui-lvgl/HistoryStore.h`). Set in its solo build. |
+| `HIST_FLASH` | Wio Tracker L1: message history also kept on the QSPI flash, across a power-off (`ui-core/HistoryLog.h`, Settings > Messages > Keep messages, off by default). Implies `HIST_ARCHIVE`'s hooks. Set in its solo builds. |
 | `MESHCORE_VERSION='"x.y"'` | Overrides the upstream MeshCore base-version string shown in diagnostics, for boards whose port hasn't been rebased onto the latest yet. Cosmetic only — doesn't change protocol behaviour. |