Notes: plain text notes on the SD card, with an editor that saves by itself (#19)

The Notes App lists the files of /notes by their first line, newest first:
n starts a note, Enter opens it, r renames its file, d deletes it after
asking, s sorts by name. A new note's file is named after its first line.

The editor wraps at spaces, 38 columns by 8 rows; Fn+arrows move through
the wrapped text, Ctrl+A and Ctrl+E go to the ends of the line. There is no
save key: the note is written five seconds after the last key, on Back, on
leaving the App, when the screen turns off and before the device powers
off. A save writes a temporary file and puts it in the note's place; a save
cut short is put back, or offered, the next time.

A note is up to 16 KB, held in one buffer reserved when it's opened: the
file is read straight into it and typing never makes it grow. A failed
allocation aborts on this device, and with IRC connected the largest free
block is about 31 KB: a first version that copied the note once on loading
restarted the device when a full note was opened with IRC connected.
Editing files of any size is #47.

The Storage App's text viewer gets `e`, which edits a text file up to 16 KB
with the same editor unless the file is read-only.

439 host tests. Checked on the device: docs/milestones/F1.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

src/services/power_service.cpp

@@ -42,6 +42,7 @@ void PowerService::applyScreen(ScreenState state) {
 }
 
 void PowerService::powerOff() {
+    if (beforePowerOff) beforePowerOff();
     auto& display = M5Cardputer.Display;
     display.wakeup();
     display.setBrightness(settings_.getInt(Setting::Brightness) * 255 / 100);

src/services/power_service.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <functional>
+
 #include "power_policy.h"
 #include "service.h"
 #include "settings.h"
@@ -24,6 +26,9 @@ class PowerService : public Service {
 
     ScreenState screen() const { return applied_; }
 
+    // Run before the device powers off: the last chance to put on the card what's only in memory.
+    std::function<void()> beforePowerOff;
+
    private:
     void applyScreen(ScreenState state);
     void powerOff();