Add novem --load tree sync with --dry-run

[bjornars]

What

Turns --load into a real tree sync and adds a --dry-run preview. --load/--dump let you round-trip a resource's file tree (plots, grids, docs, mails, jobs) to/from a local folder.

Previously --load walked the dumped folder and blindly PUT+POSTed every file — leaving stale remote files in place, rewriting unchanged ones, and reporting success regardless of what the server returned. Now it treats the local folder as the desired state:

• create files that don't exist remotely
• overwrite only files whose content differs (single-line values log the old -> new change instead of a byte count)
• delete remote files no longer present locally
• leave unchanged files untouched

--dry-run prints the planned actions without sending any request.

Notable behaviour

• Only real file_content-backed files are synced. The directory listing flattens every leaf to type="file", so selection keys on the DELETE verb in each node's actions. Read-only files (no DELETE) and virtual/computed files like notifications (POST but no DELETE) are excluded — and virtual paths that exist remotely are tracked so a stale local copy is ignored rather than re-created on every run.
• Shares (/shared/) round-trip as links created via PUT (they 405 on POST).
• Tags (/tags/) are excluded — they're managed via the -t flag.
• Each create/overwrite/delete is gated on its POST/DELETE status: failures print a FAILED line and are tallied under a N failed summary suffix instead of being silently counted as done.

Structure

The dump/load logic is shared by the vis and job APIs, so it lives in a NovemTreeSync mixin in novem/sync.py; each class supplies a base-path and a label hook. Filesystem code uses pathlib.

Testing

• New tests/test_load_sync.py covers create/overwrite/delete/unchanged, dry-run, failure reporting, single-line diffs, and the virtual/shares/tags handling.
• Full suite green (659 passed, 4 skipped); flake8 + mypy clean.
• Manually verified end-to-end against the dev backend: dump → no-op round-trip, create/overwrite/delete, share creation via PUT, and idempotency on a stale dump.

[myme]

Could've considered a --delete flag similar to rsync. But I guess this is also fine.