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/casts-src/generate.mjs

@@ -39,6 +39,10 @@ function charKey(ch) {
 const NAMED_KEYS = {
   Enter: '^M',
   Tab: '^I',
+  // Ctrl-C quits the app (Action::Quit). Used to end a cast *invisibly* — no
+  // `quit` command typed into the input — so the recording ends on the last
+  // content frame rather than a dangling, payoff-less `quit`.
+  CtrlC: '^C',
 };
 
 /** Build the autocast `keys:` list (one entry per line) for a cast's steps. */