Add live-server harness; centralise the rustls provider install

Adds a read-only example that exercises the connector against a real
Nextcloud: Login Flow v2, PROPFIND listing, the Depth:0 ETag pruning
probe, a 256KB range GET, and a server preview request. No PUT, MOVE or
DELETE, so it cannot alter a live library.

Running it against nextcloud.tourolle.paris (34.0.2) surfaced a real API
flaw rather than an example bug. The crypto provider was installed in
NextcloudBackend::new, but authentication necessarily runs *before* a
backend exists — so any caller following the documented flow panicked on
the first client build. Every entry point now goes through
`http_client()`, which installs the provider first, and auth gains
`begin_default()` for callers with no client yet. A test builds a client
without a backend to keep the regression out.

Also populates RawImage::crop from rawler's crop_area/active_area and
re-phases the CFA pattern when the crop origin is odd — cropping to the
active area without that swaps red and blue.

Login Flow v2 confirmed working against the live server: the flow URL is
issued and the poll endpoint responds. The remaining checks need a
browser approval, so they run interactively.

88 tests passing.

core/dr-sync-nextcloud/Cargo.toml

@@ -23,3 +23,4 @@ tokio = { workspace = true }
 [dev-dependencies]
 tokio.workspace = true
 env_logger.workspace = true
+dr-decode.workspace = true