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/first-project.mdx

@@ -5,10 +5,14 @@ sidebar:
   order: 2
 ---
 
+import Demo from '../../../components/Demo.astro';
+
 This is the shortest possible tour: create a table, put a row in it, and
 look at the result. Everything here is in **simple mode**, which is how the
 playground starts.
 
+<Demo src="/casts/quickstart.cast" title="The whole tour: create a table, add columns, insert a row, show the data." />
+
 When you launch `rdbms-playground` with no arguments, it opens a fresh
 temporary project for you. Type commands into the input field at the bottom
 and press <kbd>Enter</kbd> to run them. As you type, the field completes

website/src/content/docs/getting-started/modes.mdx

@@ -5,9 +5,13 @@ 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

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