fix(download): drop 5-min frontend timeout that wrongly failed completed downloads

[mickvandijke]

Summary

• Remove the 5-minute withTimeout wrapper around the download invoke() in stores/files.ts. Large downloads can legitimately exceed 5 min; the backend has no timeout and kept streaming chunks and writing the file successfully while the UI had already flipped the row to Failed: Download timed out.
• Matches the no-frontend-timeout pattern already used for uploads (confirm_upload / confirm_upload_merkle) — same rationale, same fix.
• withTimeout is still used for the two wallet-approval paths, where a 5-min cap is appropriate because the user is interactively approving a tx.

Test plan

• Download a large file whose transfer takes >5 min; confirm the row ends up Downloaded, not Failed.
• Trigger a backend download error (e.g. bad datamap) and confirm the row still flips to Failed with the backend error message — rejection still surfaces.
• Vitest: npx vitest run — 19 tests still pass.

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