feat(ui): expose the whole downloads config block in Settings

[roziscoding]

What does this PR do?

Settings already covered jack, peers, servers, and API keys in full. Of the downloads block it covered exactly one key — the unlink toggle from #79. This exposes the remaining ten, so the config file has no UI-invisible corners left.

Settings → Downloads

One form, three tiers:

• Up top, always visible — completed folder, simultaneous transfers, and the unlink switch. The settings you actually choose.
• Collapsed: "Transfer retries" — attempts, stall timeout, first and longest retry delay.
• Collapsed: "Handing files to *arr" — import check interval, import attempts, first and longest import retry delay.

Each collapsed group is labelled with how many of its values differ from jack's defaults, so a closed group still tells you whether it holds anything you set yourself. One dirty-aware Save, with Revert.

Clearing a field means "use the default"

Millisecond keys stay millisecond keys — that's what the config file holds — but each field reads its value back in human terms beside the input (1800000 → 30 min), and the placeholder is jack's default. Clearing a field sends null, and the backend drops the key from the file so the schema default applies again. completedPath has no default, so it is the one key that can't be cleared (null → 400).

A config file with no downloads block gets a "Set up downloads" flow instead of a dead end — writing that block is exactly what this form does.

Save semantics

Only unlinkImportedFiles applies immediately (the import watcher reads it per import, per #79). Everything else is read at startup, and the success message says so — including the first-save case, where nothing downloads until the server restarts.

Checklist

• Commits follow Conventional Commits
• Commits are signed (GitHub enforces this — unsigned commits are rejected)
• mise run lint:fix was run and its output is committed (no changes)
• mise run test passes — 472 pass / 0 fail. mise run test:e2e was not run: no Docker in the dev environment.
• Docs updated if behavior or configuration changed — website/reference/configuration.md, website/guide/management-ui.md

Tests

• formatDurationMs — 6 tests over the config defaults, unit boundaries, and empty values. These caught a real bug: 59999 ms rendered as 60.0 s.
• Config API — null drops a key so the default applies again; completedPath: null is rejected 400 and leaves the file untouched.
• Also exercised end to end against a running backend through the UI's own proxy: PATCH with nulls removed the keys from the config file on disk and left the rest intact.

[greptile-apps]

Greptile Summary

Adds complete Downloads configuration management to Settings.

• Exposes paths, concurrency, retry, import-watcher, and unlink settings in one form.
• Supports clearing optional values to restore schema defaults.
• Adds backend persistence semantics, validation tests, duration formatting, and documentation.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains; the previously reported post-save edit-loss path is addressed by keeping all form-editing controls disabled until both the PATCH and refresh complete.

_{Reviews (3): Last reviewed commit: "fix(ui): freeze the downloads form while..." | Re-trigger Greptile}