Seed pin/ADC/serial demos into new accounts

`_CANONICAL_DEMO_FILENAMES` now also lists `pin_demo.py`,
`adc_slider_demo.py`, and `serial_demo.py` so first-time users get
working examples for every simulator. Seeding stays idempotent — the
dst-exists guard keeps re-registration / sign-in from clobbering edits.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/editor_app/services/user_workspace.py

@@ -9,10 +9,17 @@ from editor_app import config
 DEFAULT_MAIN_PY = 'print("Hello, World!")\n'
 
 # Self-contained demos copied from shipped `workspace/code/` (stdlib + machine/neopixel/time only).
+# New accounts get a copy of each one in their own `code/` folder so the
+# editor has something to show on first login. They're treated as
+# starting points — users can edit/delete freely without affecting the
+# shipped originals.
 _CANONICAL_DEMO_FILENAMES = (
     "pattern_rainbow_demo.py",
     "pattern_twinkle_demo.py",
     "pattern_chase_demo.py",
+    "adc_slider_demo.py",
+    "pin_demo.py",
+    "serial_demo.py",
 )