Read the Downloads folder from the machine, not the server

[Frooodle]

Description of Changes

• Resolves the Downloads folder from the machine through a new getDownloadsDirectory seam, instead of asking the server for a directory it cannot see.
• Lists Downloads locally, mounts it, reads the newest PDFs in small batches and imports them.
• Classification then runs on the existing path: browser heuristic first, escalating to the connected server's AI only when unsure.
• Gates the offer on the server actually running the classification policy, so a core-flavour server cannot import files nothing will classify.
• Drops the server-side downloads-suggestion probe, which no longer has a caller.
• Corrects the copy: nothing is written back to disk, so the "processed in place, originals kept" promise is replaced.

Why: a remote server has no user Downloads folder — it answers {"directory":"","available":false,"pdfCount":0} even under an Enterprise licence — so the offer never appeared on desktop at all.

Not in scope: writing labelled bytes back to disk, and the wider processing-folder rework.

Stacked on gate-desktop-premium-surfaces — so this PR's diff includes that branch's commits and its parent's.

Open question for review: this duplicates the Downloads sweep in this PR's own base branch; the two should probably share one service.

Ran: frontend:typecheck:all, frontend:lint, frontend:format:check, full vitest, comment-lint.

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Checklist

General

• I have read the Contribution Guidelines
• I have read the Stirling-PDF Developer Guide (if applicable)
• I have read the How to add new languages to Stirling-PDF (if applicable)
• I have performed a self-review of my own code
• Every comment I added says something the code does not (guide)
• My changes generate no new warnings

Documentation

• I have updated relevant docs on Stirling-PDF's doc repo (if functionality has heavily changed)
• I have read the section Add New Translation Tags (for new translation tags only)

Translations (if applicable)

• I ran scripts/counter_translation.py

UI Changes (if applicable)

• Screenshots or videos demonstrating the UI changes are attached (e.g., as comments or direct attachments in the PR)

Testing (if applicable)

• I have run task check to verify linters, typechecks, and tests pass
• I have tested my changes locally. Refer to the Testing Guide for more details.

[github-actions]

🌐 TOML Translation Verification Summary

🔄 Reference Branch: pr-branch

📃 File Check: en-US/translation.toml

1. Test Status: ✅ Passed
2. Test Status: ✅ Passed
3. Test Status: ✅ Passed

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✅ Overall Check Status: Success

Thanks @Frooodle for your help in keeping the translations up to date.

[github-actions]

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