scratch: a drawer of notes taken in passing, kept as documents later

Q lowers the scratchpad from under the bar over whatever is on screen,
as the desktop lowers its own. Its notes have no name or place; each is
written in the vim editor and saves itself. K (:keep [title]) makes the
note under the cursor a document in the folder being browsed, titled by
its first line, and takes it off the pad. :scratch <text> jots one from
the command line. The notes ride along in backups and whole-workspace
exports.

The card description's editor is now TextEditor, shared by both.

migrations/0026_scratch.sql

@@ -0,0 +1,12 @@
+-- The scratchpad: notes written down in passing, before it is clear where they
+-- belong. They live outside the tree on purpose — no space, no folder, no
+-- name — so taking one costs nothing. Keeping one is making it a document,
+-- which takes it out of here.
+CREATE TABLE scratch_notes (
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    body       TEXT    NOT NULL DEFAULT '',
+    created_at INTEGER NOT NULL,
+    updated_at INTEGER NOT NULL
+);
+
+CREATE INDEX idx_scratch_notes_updated ON scratch_notes (updated_at);