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.

0 files changed, 0 insertions, 0 deletions