Add 'Reset demos' button to refresh canonical demo files

Existing accounts (including admin) seeded before new demos shipped
had no easy way to pull in the latest copies — the registration-time
seeder is intentionally non-destructive. The new badge action fetches
src/static/bundled-demos/manifest.json, confirms the overwrite, and
re-copies each canonical demo into code/. Open tabs of those files are
refreshed in place so the user sees the new content immediately.

src/static/bundled-demos/ ships the six canonical files plus the
manifest so this works in local mode and on a static-only host. The
Dockerfile now mirrors workspace/code/<demo>.py into bundled-demos/
during the image build, keeping the two locations in sync.

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

src/static/bundled-demos/serial_demo.py

@@ -0,0 +1,90 @@
+"""Serial in/out demo.
+
+When this script runs, a "Serial monitor" pane appears below the editor.
+
+Try this:
+  * type   hello       and press Enter            -> Python echoes "echo: hello"
+  * type   color red                              -> the strip turns red
+  * try    color 0,128,255                        -> any (r,g,b) tuple works
+  * type   off                                    -> strip blanks
+  * type   bye                                    -> script exits cleanly
+
+Anything Python `write()`s to the UART shows up in green; what you type back
+is shown in white.
+"""
+
+import time
+
+from machine import Pin, UART
+from neopixel import NeoPixel
+
+
+NUM_LEDS = 16
+strip = NeoPixel(Pin(5, Pin.OUT), NUM_LEDS)
+uart = UART(0, baudrate=115200)
+
+PALETTE = {
+    "red": (255, 0, 0),
+    "green": (0, 255, 0),
+    "blue": (0, 0, 255),
+    "white": (200, 200, 200),
+    "purple": (160, 0, 200),
+    "orange": (255, 110, 0),
+}
+
+
+def fill(color):
+    strip.fill(color)
+    strip.write()
+
+
+def parse_color(arg):
+    arg = arg.strip().lower()
+    if arg in PALETTE:
+        return PALETTE[arg]
+    parts = [p for p in arg.replace(",", " ").split() if p]
+    if len(parts) == 3:
+        try:
+            return tuple(max(0, min(255, int(p))) for p in parts)
+        except ValueError:
+            return None
+    return None
+
+
+uart.write("ready. commands: color <name|r,g,b> | off | bye\n")
+fill((0, 0, 0))
+
+running = True
+while running:
+    line = uart.readline()
+    if line is None:
+        time.sleep(0.05)
+        continue
+
+    text = line.decode("utf-8", errors="replace").strip()
+    if not text:
+        continue
+
+    if text == "bye":
+        uart.write("goodbye!\n")
+        running = False
+        break
+
+    if text == "off":
+        fill((0, 0, 0))
+        uart.write("strip off\n")
+        continue
+
+    if text.startswith("color"):
+        rest = text[len("color"):].strip()
+        color = parse_color(rest) if rest else None
+        if color is None:
+            uart.write("usage: color <name> | color r,g,b\n")
+        else:
+            fill(color)
+            uart.write(f"strip = {color}\n")
+        continue
+
+    uart.write(f"echo: {text}\n")
+
+fill((0, 0, 0))