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

site/data/keys.toml

@@ -291,7 +291,7 @@ rows = [
     ["; .", "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"],
 ]
 
@@ -360,6 +360,7 @@ rows = [
     ["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: é"],
     ["`", "done: it saves by itself"],