fish_greeting installer asks for the API origin and probes the backend

install.fish now:
- asks for the API origin (default https://j621.rainbow-herring.ts.net) and
  an optional token when run interactively, or takes --url/--token;
- writes ~/.config/j621Greeting/config.fish with mode 600 (it may hold a
  token), keeps an existing config unless --force/--url is given, and
  --no-prompt runs fully unattended;
- verifies the setup: /health must answer 'ok' and a fastfetch random roll is
  attempted, reporting the API's own message when it finds nothing; exits
  non-zero when the backend is unreachable so scripts notice.

README documents the prompts/flags, the chmod 600 config, and that the
greeting is designed to run without a token (guest mode: unsigned links,
guest-visible items only) with a token adding signed links and hidden items.

Tested with fish 4.9.3 in throwaway HOME dirs: flag-driven install with a
token, interactive install with a piped origin and no token, re-run keeping
the existing config, and an unreachable backend exiting 1.

README.md

@@ -7,6 +7,8 @@ Self-hosted media library and e621 archive manager, rebuilt as a **React SPA + D
 ```
 backend/    Django 6 + DRF API (MariaDB + Redis via docker compose)
 frontend/   Vite + React + TypeScript SPA
+deploy/     Docker images, compose variants and Tailscale serve configs
+extras/     shell integrations (fish_greeting with fastfetch)
 ```
 
 ## Development
@@ -73,6 +75,8 @@ curl -H "Authorization: Token <token>" \
   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.
+- A ready-made shell greeting that uses this endpoint lives in
+  [`extras/fish_greeting/`](extras/fish_greeting/README.md).
 
 ## Licence