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.cpp

@@ -204,6 +204,7 @@ void FileViewer::close() {
     file_.reset();
     pager_.reset();
     confirm_.reset();
+    message_.clear();
     std::vector<std::string>().swap(shown_);
     std::vector<std::string>().swap(details_);
     packets_.setCount(0);
@@ -216,6 +217,11 @@ void FileViewer::show(Mode mode) {
     rowsFrom_ = -1;
 }
 
+void FileViewer::say(const std::string& text) {
+    message_ = text;
+    messageMs_ = millis();
+}
+
 std::string FileViewer::title() const { return files::fitName(files::baseName(path_), 24); }
 
 void FileViewer::scroll(int lines) {
@@ -314,6 +320,10 @@ bool FileViewer::onKey(const KeyEvent& e) {
 }
 
 bool FileViewer::update(uint32_t) {
+    if (!message_.empty() && millis() - messageMs_ >= 4000) {
+        message_.clear();
+        return true;
+    }
     if (!scan_) return false;
     if (scan_->done && !scanShown_) {
         scanShown_ = true;
@@ -387,6 +397,7 @@ void FileViewer::draw(Canvas& c) {
             }
             right = formatBytes(size_);
             keys = std::string("Tab: ") + (mode_ != base_ ? "back" : mode_ == Mode::Text ? "hex" : "text") + "  t: top  b: end";
+            if (mode_ == Mode::Text) keys += "  e: edit";
             break;
         }
         case Mode::Gpx: {
@@ -452,7 +463,8 @@ void FileViewer::draw(Canvas& c) {
     c.setTextDatum(top_right);
     c.drawString(right.c_str(), area.w - 3, area.y + 1);
     c.setTextDatum(top_left);
-    c.drawString(keys.c_str(), 4, area.y + area.h - 9);
+    if (!message_.empty()) c.setTextColor(theme::kWarning);
+    c.drawString(message_.empty() ? keys.c_str() : message_.c_str(), 4, area.y + area.h - 9);
 }
 
 }  // namespace roro