mara on #764:9897: "host the fluffy chat app at / as follow up?"
Moves fluffychat-web from the bare-domain sub-path
(`<hive>/matrix/`) to the matrix sub-domain root
(`matrix.<hive>/`). Follow-up to #764 (matrix vhost itself), per
mara's gateway-architecture verdict (sub-domain for external standard
apps, sub-path for hyperhive-internal). Stacked on
`atlas/747-matrix-behind-gateway` — depends on #764 landing first.
## Mechanics
**hive-matrix.nix:**
- Drop `flutterBuildFlags = [ "--base-href" "/matrix/" ]` from
`fluffychat-web-fixed`. Upstream default `--base-href "/"` is correct
at sub-domain root.
- Update option docs to reflect new mount point.
**hive-gateway.nix:**
- `$matrix_spa_target` map target flips from `/matrix/index.html` →
`/index.html` (sub-domain root now).
- New `<hive>/matrix/*` location: `rewrite ^/matrix/(.*)$
matrix.<hive>/$1 permanent;` — 301 redirect preserves bookmark +
deep-link compatibility for `<hive>/matrix/#/rooms/...` URLs during
the transition.
- `<hive>/matrix/config.json` location removed (moved to `/config.json`
on the matrix vhost).
- Matrix vhost (#764) gains `/` location: serves fluffychat dist as
static files with the Accept-header SPA fallback (`/_matrix/`
proxying to tuwunel keeps working via nginx longer-prefix-wins
precedence). When `gui.enable = false`, `/` returns 404 cleanly.
- Matrix vhost gains `= /config.json` for the FluffyChat boot-config
pre-fill (#736).
## Verified
```
vhosts: ["_", "forge.test.local", "matrix.test.local"]
bare locations: ["/", "/matrix/", "= /.well-known/matrix/client",
"= /.well-known/matrix/server"]
matrix vhost locations: ["/", "/_matrix/", "= /config.json"]
/matrix/ extraConfig: "rewrite ^/matrix/(.*)$ http://matrix.test.local/$1 permanent;"
matrix vhost / alias: /nix/store/...fluffychat-web-2.6.0/
```
Full container toplevel builds clean.
## Risk
Medium. Two breaking changes for operators:
1. **Bookmark migration**: `http://<hive>/matrix/#/rooms/...` 301s
to `http://matrix.<hive>/#/rooms/...`. Browser bookmarks +
shared links keep working via the redirect; can be cleaned up
once it's been in the wild long enough.
2. **fluffychat-web dist hash changes**: dropping the
`--base-href "/matrix/"` flag changes the derivation hash, so
`gui.package` rebuilds even though the source is the same.
Operators on substitute caches will fetch the new dist; building
from source takes the same time as before.
The `.well-known/matrix/{client,server}` delegation (already
advertising `matrix.<hive>` per #764) means matrix clients
auto-discover the new location — no client config change needed.
## Sequencing
**Depends on #764** — needs the matrix vhost to host the new `/`
location. Merge after #764 lands + soaks. If #764 changes shape
during review I'll rebase + force-push.
Closes#772.
df549ed2a5