Gate the remote-interval poll on a root ETag check (#189)

[gnacho]

Closes #189.

Problem

The periodic remote-interval poll ran a full nextcloudcmd reconciliation of every folder every N minutes regardless of whether anything changed, even after #183/#185 (which only apply when the server offers notify_push). On the large folders this is a full tree re-discovery (tens of thousands of progress events per cycle) - exactly what the user sees as "it keeps syncing even though no files changed". With remote_interval_minutes=10 and no push on the server, Fotos (~65k files), Mi Nube (~16k), Zbox (~11k) re-scan every cycle.

Fix (mirrors the official client)

The official client does a cheap ETag poll before deciding to reconcile: RequestEtagJob is a single PROPFIND with Depth: 0 asking only for <getetag/> (src/libsync/networkjobs.cpp), and Folder::etagRetrieved compares it against _lastEtag; the full sync runs only when the root ETag changed (folder.cpp:426).

Changes:

• NextcloudApi::root_etag: a PROPFIND Depth:0 for <getetag/> on the folder root (new PROPFIND_ETAG_BODY), returning the trimmed ETag or None.
• EtagProbe / ProductionEtagProbe (mirrors the HealthProbe pattern from Mark folder offline after N consecutive failed syncs regardless of error class #179), wired in production via with_etag_probe.
• The engine tracks a per-folder etag_slot (shared Arc<Mutex<Option<String>>>). On a pure RemoteInterval run it reads the root ETag and, if unchanged from the last run, reports a clean SyncOutcome::Success without spawning nextcloudcmd. A changed ETag (or first run / unavailable ETag) reconciles as before.
• Non-interval triggers (manual, inotify, startup, remote push, network-restored, resume, retry, recovery) always reconcile - etag_gate_applies only accepts a sole RemoteInterval.

Testing

691 tests (6 new), clippy clean, fmt applied.

New tests: root_etag_reads_the_etag_with_depth_zero, root_etag_absent_body_yields_none, parse_root_etag_returns_trimmed_text, unchanged_etag_skips_the_interval_run (asserts nextcloudcmd not spawned), changed_etag_reconciles_the_interval_run, manual_run_ignores_the_etag_gate.