feat(website): casts for first-project/modes/undo-redo; quit invisibly via Ctrl-C

Three more casts on "doing" pages:
- first-project reuses the quickstart cast (the create→insert→show tour)
- modes (new): a simple command, then `mode advanced` where the same command
  also prints "Executing SQL: …" (the teaching echo — "learn the SQL underneath")
- undo-redo (new): insert two rows, `undo` (Y-confirm modal) backs one out,
  `redo` restores it

Also fix the cast endings (review feedback): scripts ended by typing a `quit`
command, which — once the trim drops the shell exit — left a dangling "quit" in
frame with no payoff. End every cast with Ctrl-C instead (the app's quit key,
KeyCode::Char('c')+CTRL): it types nothing, so the cast ends cleanly on the
last content frame. Generator gains a `CtrlC` key; all six casts regenerated.

Convert the three pages to .mdx and embed. Build clean (26 pages); 6 casts.

website/src/content/docs/using-the-playground/undo-and-history.mdx

@@ -5,9 +5,13 @@ sidebar:
   order: 5
 ---
 
+import Demo from '../../../components/Demo.astro';
+
 Every change you make is safe to undo — the playground snapshots the whole
 project before each one.
 
+<Demo src="/casts/undo-redo.cast" title="Undo a row back out, then redo it — each step asks for confirmation." />
+
 ## Undo and redo
 
 ```rdbms