fix: web UI and src.common bugs (wifi wrong-password, plugin icon, starlark toggle, API caching, scroll/logo/font helpers) (#646)

- wifi: keep the "wrong_password:" prefix through the restore/AP fallback so
  the UI's incorrect-password prompt fires again.
- /plugins/installed returns the manifest's icon (string only).
- /starlark/apps/<id>/toggle coerces `enabled` and delegates to
  _toggle_starlark_app (disk before memory, no KeyError, "false" is false).
- /api/v3/ JSON GETs are sent Cache-Control: no-store; non-JSON keeps 5s.
- ScrollHelper.set_scrolling_image converts non-RGB input (alpha onto black);
  create/set_scrolling_image reset last_update_time like reset_scroll.
- LogoHelper backs off a failed download per path for
  MISSING_LOGO_RECHECK_SECONDS; cleared on invalidate/clear_cache.
- refresh_placeholder_timestamp saves atomically.
- FontManager.clear_cache / _clear_plugin_font_cache bump cache_generation.
- Odds manager: per-game logs to DEBUG; JSON decode error caught before
  RequestException (same cooldown).
- element_style mangled continuations; startup validator skips null plugin
  blocks and reuses the controller's discovery.
- src/common/README lists frame_timing, json_body, render_gate.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/logo_downloader.py

@@ -235,7 +235,10 @@ def refresh_placeholder_timestamp(filepath: Path) -> bool:
         metadata = PngInfo()
         metadata.add_text(PLACEHOLDER_MARKER, str(time.time()))
         with Image.open(filepath) as img:
-            img.copy().save(filepath, "PNG", pnginfo=metadata)
+            image = img.copy()
+        # Atomically, like every other logo write: a renderer can open this
+        # file at any moment, and an in-place save exposes a truncated PNG.
+        save_png_atomically(image, filepath, pnginfo=metadata)
         return True
     except Exception:
         logger.debug("Could not refresh placeholder timestamp for %s", filepath,