Rethink the AI dropdown in the Machinery panel

Now that automatic LLM suggestions exist (see https://github.com/mozilla/pontoon/issues/4388), the AI dropdown placement and shape deserve a second look:

• Which suggestions get it? Right now only Google Translate. TM matches, Microsoft Translator, and Caighdean results are equally refinable; composed suggestions are excluded too (#4235) and so are OpenAI ChatGPT suggestions.
• Position when sources are merged. Identical translations from multiple sources are collapsed into one row with a merged source list, so the control's position shifts with where Google Translate lands in translation sources. Perhaps it belongs on the suggestion row itself, not inside one source's label.
• Dropdown or button? If Rephrase characteristic is used significantly more often than Make formal / informal, we may not need a menu — a single "Refine with AI" button would be simpler and one click shorter.
• Beyond Machinery? Refining an existing translation in the History panel is arguably the more valuable case — the string already has a human version to improve.

Worth reviewing the LLM Dropdown Select / LLM Translation Copied UX action data before redesigning, to see which characteristics are actually used.

Review the LLM Dropdown Select and LLM Translation Copied UX action data before comparing the current Machinery-panel behavior with the History-panel case. Use issues #4388 and #4235 for related context; done requires an agreed direction for suggestion coverage, placement, and whether the control remains a dropdown or becomes a button.