feat(groups): host-based forge sources with optional display name

- forgeSources entries are now { host, type, name? }: the repository URL's
  hostname is matched case-insensitively against host (github.com for GitHub,
  distinct hosts for multiple Gitea instances); the footer label is the
  optional name, falling back to the host
- GroupEditor renders one row per source: host input + type select + optional
  display name (grid layout); hostname validation mirrors the server
- fix: apiFetch sends Content-Type: application/json — h3's readBody only
  parses JSON bodies with that header, so every PUT/POST from the refactored
  console arrived as a raw string and failed with 'groups must be an array'
- hardening: readJsonBody (admin + actions) JSON-parses string bodies so curl
  and older clients without the content-type header still work
- regression test: groups PUT without content-type + forgeSources round-trip
- docs: groups.md/message-format.md (en/zh), AGENTS.md, config.example.yaml

docs/guide/message-format.md

@@ -29,7 +29,7 @@ Event-specific emoji are added by the formatters; per-group `Group.emoji` (defau
 
 ## Forge Source Label
 
-With `Group.forgeLabel` (default false) the message footer additionally names the source forge — GitHub, the Gitea instance hostname (hyperlinked), or a plain `Custom` — so events from different forges can be told apart. See [Groups → Forge Source Label](./groups#forge-source-label).
+With `Group.forgeSources` (a list of `{ host, type, name? }` entries) the message footer names the matching source — the first entry whose type (`github` / `gitea`) matches the event's provider and whose host matches the repository URL's hostname, labeled with its optional `name` (falling back to the host) — so events from different forges can be told apart. See [Groups → Forge Source Label](./groups#forge-source-label).
 
 ## In-Place Updates