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/getting-started/modes.mdx

@@ -0,0 +1,64 @@
+---
+title: Simple and advanced modes
+description: How the playground's two input modes differ, how to switch, and the one-line escape hatch.
+sidebar:
+  order: 3
+---
+
+import Demo from '../../../components/Demo.astro';
+
+The playground has two input modes. You can do everything a beginner needs
+in **simple mode**, and reach for **advanced mode** when you want full SQL.
+
+<Demo src="/casts/modes.cast" title="The same command in both modes — advanced mode also shows the SQL it runs for you." />
+
+## Simple mode (the default)
+
+Simple mode is a friendly, keyword-based command language designed for
+learning. Commands read close to English:
+
+```rdbms
+create table authors with pk author_id(serial)
+show data authors
+```
+
+Simple mode accepts these learning commands plus the app-level commands
+(like `save`, `undo`, and `help`). If you type raw SQL here, the playground
+gently points you at advanced mode instead of failing silently.
+
+## Advanced mode (SQL)
+
+Advanced mode accepts standard SQL — `SELECT`, `INSERT`, `CREATE TABLE`, and
+more — alongside the same app-level commands:
+
+```sql
+select title, published from books where published >= 2000 order by published;
+```
+
+Switch modes with the `mode` command:
+
+```rdbms
+mode advanced
+mode simple
+```
+
+The mode you leave a project in is remembered and restored the next time you
+open it, so a project set up for SQL practice reopens in advanced mode.
+
+## The one-line escape
+
+When you are in simple mode and want to run a single SQL statement without
+switching, prefix the line with a colon:
+
+```rdbms
+:select count(*) from books
+```
+
+That runs just this one line as SQL; you stay in simple mode afterwards.
+
+## Seeing the SQL behind a command
+
+When you run a simple-mode command in advanced mode, the playground prints
+the equivalent SQL beneath your command. It is a built-in way to learn how
+the friendly commands map onto real SQL — the same statements you could type
+yourself in advanced mode.