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/font_manager.py

@@ -426,6 +426,9 @@ class FontManager:
         keys_to_remove = [key for key in self.font_cache.keys() if key.startswith(f"{plugin_id}::")]
         for key in keys_to_remove:
             del self.font_cache[key]
+        if keys_to_remove:
+            # Font objects someone may hold were dropped; see cache_generation.
+            self.cache_generation += 1
 
     @deprecated("3.7.0")
     def get_plugin_fonts(self, plugin_id: str) -> List[str]:
@@ -732,6 +735,9 @@ class FontManager:
         """Clear font and metrics cache."""
         self.font_cache.clear()
         self.metrics_cache.clear()
+        # Holders of derived caches (layout fits, font usage) key off this;
+        # without the bump they kept serving results for the dropped fonts.
+        self.cache_generation += 1
         logger.info("Font cache cleared")
 
     @deprecated("3.7.0", "read font_catalog")