Notes: edit a text file of any size (#47)

The editor held the whole note in memory and stopped at 16 KB. It now keeps
a window of the file around the cursor, and the rest on the card as a list
of pieces (notes::NoteDocument). Memory with a note open is what it was.

Up to 64 KB a save rewrites the file, as before. Above, the five-second
save appends what changed to <note>.edit, and the file is rewritten on
leaving the note, with a progress bar. After a power cut, opening the note
picks the edit up where it was saved; a rewrite cut short is finished or
dropped, never half applied.

Also: Ctrl with Fn+Up/Down go to the start and end of the note; the
consoles' `key` command takes ctrl-, alt- and shift-; the Storage App's
`e` no longer refuses a big file.

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

src/apps/storage_app.cpp

@@ -322,7 +322,6 @@ bool StorageApp::onKey(const KeyEvent& e) {
     if (view_ == View::Viewer && viewer_.showingText() && e.key == Key::Char && (e.ch == 'e' || e.ch == 'E')) {
         // Edit it (Q146), if the rules and its size allow.
         std::string path = viewer_.path(), why = ops_.whyReadOnly(path, false);
-        if (why.empty() && viewer_.size() > notes::NoteText::kMaxBytes) why = "Too big to edit: 16 KB at most";
         if (why.empty()) {
             viewer_.close();
             why = noteEditor_.open(path);