Port the fish_greeting shell greeting to the new API

The original script (J621-Django/extras/fish_greeting system) downloaded
image bytes from /random/?rating=X and read the X-File-* headers. The new
API answers with JSON and a signed link, so the greeting now:

- asks /api/random/?fastfetch=1&rating=<mode> for JSON;
- parses url/j_id/filename/md5 with jq, python3, or a grep/sed fallback;
- downloads the signed link and keeps the original display path (fastfetch
  kitty/kitty-icat logos, gifsicle preprocessing for GIFs, recursion guard,
  logging with rotation, greeting_mode 0/1/2 = NSFW/SFW/Questionable);
- is configured through ~/.config/j621Greeting/config.fish or universal
  variables (J621_BASE, J621_WEB, J621_TOKEN, J621_FASTFETCH_CONFIG) instead
  of a hardcoded host, and prints the /detail/<J-ID> link on J621_WEB;
- reports the API's own 404 message when a rating has no images, and keeps
  curl quiet so failures do not spill into the greeting.

extras/fish_greeting/ contains the function, an install.fish (copies it into
the fish functions dir, creates the config once, checks dependencies) and a
README with the old-vs-new table and troubleshooting. The existing
gm-switch helper and .desktop launchers keep working (same mode file).

Tested with fish 4.9.3: syntax check on every file, guest and token runs
against the dev API (unsigned vs signed URLs), the empty-rating and
unreachable-API paths, and the grep/sed fallback with jq and python3
unavailable.

README.md

@@ -69,8 +69,10 @@ curl -H "Authorization: Token <token>" \
 - `url` is absolute, and signed for authenticated callers, so fastfetch can
   load it without headers. Guests get an unsigned URL and only see
   guest-visible items.
-- `/random` and `/random/` are aliases of `/api/random/` for scripts; 404 when
-  nothing matches the filters.
+- `/random` and `/random/` are aliases of `/api/random/` for scripts. Behind
+  the bundled nginx those aliases negotiate on `Accept`: browsers get the SPA
+  page, requesters like curl/wget/fastfetch get the JSON. `/api/random/` is
+  the unambiguous path for scripts; 404 when nothing matches the filters.
 
 ## Licence