Add Portal-style Still Alive demo and split build envs

- src/still_alive: timed console text, typed scrolling credits and ASCII art,
  laid out for 512x256 (41x31 console, 41x9 credits, 41x20 picture).
  Plays placeholder content unless imported.h exists.
- tools/import_still_alive.py: generates the git-ignored imported.h (text,
  timing, art, credits) from SDSkyKlouD/still-alive-web.
- platformio.ini: envs still_alive (default) and esp32dev (test images,
  moved to src/test_images).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -24,3 +24,6 @@ $RECYCLE.BIN/
 # Editors
 *.swp
 *~
+
+# Imported still_alive text (tools/import_still_alive.py)
+src/still_alive/imported.h