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>

test/test_font_manager.py

@@ -137,6 +137,24 @@ class TestCacheLifecycle:
         assert fm.cache_generation == gen_before + 1
         assert not fm.font_cache
 
+    def test_clear_cache_bumps_generation(self, fm):
+        # Layout contexts and font-usage results are keyed off
+        # cache_generation; clear_cache used to drop the fonts without
+        # telling them.
+        gen_before = fm.cache_generation
+        fm.clear_cache()
+        assert fm.cache_generation == gen_before + 1
+
+    def test_clearing_a_plugins_cached_fonts_bumps_generation(self, fm):
+        fm.font_cache["demo::tiny_8"] = object()
+        gen_before = fm.cache_generation
+        fm._clear_plugin_font_cache("demo")
+        assert "demo::tiny_8" not in fm.font_cache
+        assert fm.cache_generation == gen_before + 1
+        # Nothing to drop, nothing to rebuild.
+        fm._clear_plugin_font_cache("demo")
+        assert fm.cache_generation == gen_before + 1
+
 
 class TestPluginFonts:
     """plugin:// sources resolve against the plugin's own directory, which