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/display_controller.py

@@ -318,13 +318,19 @@ class DisplayController:
             except Exception as e:
                 logger.warning("Could not enable plugin health/resource monitoring: %s", e)
 
+            # Discover plugins. Before the plugin checks so they can reuse the
+            # list: each discover_plugins() call rescans the plugins directory
+            # and logs every plugin again.
+            discovered_plugins = self.plugin_manager.discover_plugins()
+            logger.info("Discovered %d plugin(s)", len(discovered_plugins))
+
             # Only the plugin checks: validate_all() above has run the rest,
             # and running it again logged every config warning twice.
             try:
                 from src.startup_validator import StartupValidator
                 validator = StartupValidator(self.config_manager, self.plugin_manager,
                                              cache_manager=self.cache_manager)
-                validator._validate_plugins()
+                validator._validate_plugins(discovered_plugins=discovered_plugins)
                 for warning in validator.warnings:
                     logger.warning("Plugin validation warning: %s", warning)
                 if validator.errors:
@@ -333,10 +339,6 @@ class DisplayController:
             except Exception as e:
                 logger.warning("Plugin validation could not be completed: %s", e)
 
-            # Discover plugins
-            discovered_plugins = self.plugin_manager.discover_plugins()
-            logger.info("Discovered %d plugin(s)", len(discovered_plugins))
-
             # Check for on-demand plugin filter from cache
             on_demand_config = self.cache_manager.get('display_on_demand_config', max_age=3600)
             enabled_plugins = self._select_startup_plugins(discovered_plugins, on_demand_config)