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>

web_interface/app.py

@@ -570,10 +570,16 @@ def add_security_headers(response):
         response.headers['Cache-Control'] = 'public, max-age=31536000, immutable'
         response.headers['Expires'] = (datetime.now() + timedelta(days=365)).strftime('%a, %d %b %Y %H:%M:%S GMT')
     elif request.path.startswith('/api/v3/'):
-        # Short cache for API responses (5 seconds) to allow for quick updates
-        # but reduce server load for repeated requests
         if request.method == 'GET' and 'stream' not in request.path:
-            response.headers['Cache-Control'] = 'private, max-age=5, must-revalidate'
+            if response.mimetype == 'application/json':
+                # JSON is live state. A cached copy made a fetch() right after
+                # an install, toggle or Wi-Fi connect show the state from
+                # before it for up to the cache lifetime.
+                response.headers['Cache-Control'] = 'no-store'
+            else:
+                # Files served through the API (plugin web UI assets, backup
+                # downloads) keep the short cache.
+                response.headers['Cache-Control'] = 'private, max-age=5, must-revalidate'
     else:
         # No cache for HTML pages to ensure fresh content
         response.headers['Cache-Control'] = 'no-cache, no-store, must-revalidate'