feat(invoke): show "Use as Ref Image" button for any image when InvokeAI is configured

[lstein]

Summary

• The "Use as Ref Image" button only needs the image file itself (it uploads the file and asks InvokeAI to use it as a reference) — it doesn't need any recallable generation parameters in the image metadata.
• So it is now exposed from every metadata-drawer path whenever an InvokeAI backend URL is configured: EXIF-only images, images with no metadata at all, scalar-only custom-workflow InvokeAI images, and images whose Invoke metadata fails to parse.
• The Recall / Remix buttons still require recallable Invoke parameters and remain gated on a successful parse — only the standalone use-ref button follows this broader rule.

Implementation notes

• Extracted the single-button markup into _USE_REF_BUTTON_HTML and added a public use_ref_button_html() helper that wraps it in its own .invoke-recall-controls container.
• format_invoke_metadata now appends the standalone helper on the scalar-only and validation-error branches when show_recall_buttons=True; the fully-parsed path still renders the three-button group in a single container.
• format_metadata computes invokeai_configured once, dispatches to the Invoke/EXIF/no-metadata renderer as before, and appends the standalone button for the non-Invoke paths when configured.

Test plan

• ruff check photomap tests passes.
• pytest tests/backend — 117 passed (new test_metadata_formatting.py covers the three dispatch paths × InvokeAI configured/not; new formatter tests cover scalar-only / validation-error + standalone helper).
• npm test — 233 passed.
• Manual: browse an album containing a non-Invoke (phone camera) image with InvokeAI configured and verify the "Use as Ref Image" button appears in the drawer and uploads the file.
• Manual: clear the InvokeAI URL in settings and verify no recall buttons appear on any image.

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