fix: diagnosable translation errors and icon hidden after scroll

Translation error ("設定値が不正です") diagnosis
- The INVALID_SETTINGS code collapsed two unrelated failures into one
  opaque message: (a) no model configured in the background settings
  cache, and (b) HTTP 400/403 from Ollama with the actual reason in the
  response body. Verified against a live Ollama server: the configured
  cloud models (glm-5.3:cloud etc.) stream fine from a plain page, so
  the generic message gave no clue which path fired.
- ollama-client.js: read the Ollama error body on non-OK responses and
  append its message ("model is required" etc.) to the thrown error
- message-router.js: no-model-configured now has its own explicit
  message ("モデルが設定されていません...") and concrete detail reasons
  are appended to the base message in the popover, e.g.
  設定値が不正です。... (Ollama /api/chat failed: 400 - ...)

Action icon disappearing
- The scroll handler hid the action button whenever the popover was
  hidden, even with a live selection, leaving no way to translate until
  the user re-selected. It now repositions the icon while a selection
  exists and only hides it when the selection is gone (Design 5.4).

Results: 73 passed, 0 failed. dist rebuilt.

1 files changed, 10 insertions, 2 deletions

diff --git a/src/content/content.js b/src/content/content.js
index fa5f416..07e6cdc 100644
--- a/src/content/content.js
+++ b/src/content/content.js
@@ -630,12 +630,20 @@
   document.addEventListener('mouseup', onSelectionEvent, { passive: true });
   document.addEventListener('keyup', onSelectionEvent, { passive: true });
 
-  // Reposition or hide UI when the viewport changes (Design 5.4).
+  // Reposition the UI while the viewport changes (Design 5.4). Keep the
+  // icon visible whenever a selection exists - hiding it on scroll left
+  // users without any way to translate until they re-selected.
   window.addEventListener(
     'scroll',
     () => {
       if (popover.state === POPOVER_STATES.HIDDEN) {
-        actionButton.hide();
+        const selection = readSelection();
+        if (selection) {
+          currentSelectionRect = selection.rect;
+          actionButton.show(selection.rect);
+        } else {
+          actionButton.hide();
+        }
         return;
       }
       const selection = readSelection();