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

lib/core/src/app_keys.h

@@ -255,7 +255,7 @@ inline constexpr KeyHelp kViewerText[] = {
     {"; .", "a line up, down"},
     {", /", "a page up, down"},
     {"t  b", "the top, the end"},
-    {"e", "edit it (up to 16 KB)"},
+    {"e", "edit it"},
     {"Tab", "the file as hex, or back"},
 };
 
@@ -310,6 +310,7 @@ inline constexpr KeyHelp kNotesEditor[] = {
     {"Tab", "two spaces"},
     {"Fn ; . , /", "move the cursor"},
     {"Alt Fn ; .", "a page up, down"},
+    {"Ctrl Fn ; .", "start, end of the note"},
     {"Ctrl a  e", "start, end of the line"},
     {"opt ' e", "an accent: \xC3\xA9"},
     {"`", "done: it saves by itself"},