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/common/scroll_helper.py

@@ -254,6 +254,9 @@ class ScrollHelper:
         now = time.time()
         self.scroll_start_time = now
         self.last_progress_log_time = now
+        # The position just went back to 0; the first update must not advance
+        # it by however long the helper sat idle (off-screen) before this.
+        self.last_update_time = now
         self.logger.info(
             "Dynamic duration target set to %ds (min=%ds, max=%ds, buffer=%.2f)",
             self.calculated_duration,
@@ -776,6 +779,18 @@ class ScrollHelper:
             self.clear_cache()
             return
         
+        # Every frame is cut from cached_array with Image.frombytes('RGB', ...),
+        # which reads a 4-channel (RGBA) array as garbage and raises on a
+        # 1-channel (L) one. Transparent pixels go to black, the panel's
+        # background, rather than to whatever colour hides under the alpha.
+        if image.mode != 'RGB':
+            if 'A' in image.mode or 'transparency' in image.info:
+                rgba = image.convert('RGBA')
+                image = Image.new('RGB', rgba.size, (0, 0, 0))
+                image.paste(rgba, (0, 0), rgba)
+            else:
+                image = image.convert('RGB')
+
         # Set the cached image
         self.cached_image = image
         
@@ -802,6 +817,9 @@ class ScrollHelper:
         self.scroll_start_time = now
         self.last_progress_log_time = now
         self.last_step_time = now  # Initialize step timer for frame-based scrolling
+        # The position just went back to 0; the first update must not advance
+        # it by however long the helper sat idle before this image arrived.
+        self.last_update_time = now
         
         self.logger.debug("Set scrolling image: %dx%d, total_scroll_width=%d", 
                          image.width, image.height, self.total_scroll_width)