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

@@ -0,0 +1,43 @@
+---
+title: Undo, redo & history
+description: Step back from any change, and replay a recorded session.
+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
+undo
+```
+
+```rdbms
+redo
+```
+
+Each asks you to confirm, naming the exact command being undone or
+re-applied. Redo is available until you make a new change. (How snapshots
+work is explained in [Concepts](/concepts/projects-and-storage/).)
+
+Starting with `--no-undo` turns this off for the session — no snapshot is
+taken before each change, and `undo`/`redo` report that undo is off.
+
+## History and replay
+
+Every command you run is recorded, in order, in the project's `history.log`.
+You can re-run a saved sequence of commands from a file:
+
+```rdbms
+replay path/to/commands
+```
+
+Replay runs each non-blank, non-`#`-comment line, stopping at the first error
+(relative paths resolve under the project directory). It re-applies the
+schema- and data-changing commands and skips app-level ones.