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/apps/file_viewer.h

@@ -30,6 +30,12 @@ class FileViewer {
     bool update(uint32_t nowMs);    // true: draw again
     void draw(Canvas& c);
 
+    // For `e` in the Storage App: is this a text small enough to edit (Q146)? And a line to say why not.
+    bool showingText() const { return mode_ == Mode::Text; }
+    const std::string& path() const { return path_; }
+    uint32_t size() const { return size_; }
+    void say(const std::string& text);
+
    private:
     enum class Mode { Text, Hex, Gpx, Pcap, Packet, Ota };
     static constexpr int kRows = 8;
@@ -62,6 +68,8 @@ class FileViewer {
     std::vector<std::string> details_;
     int detailsTop_ = 0;
     std::unique_ptr<DialogModel> confirm_;
+    std::string message_;
+    uint32_t messageMs_ = 0;
 };
 
 }  // namespace roro