chore: remove the dashboard's pre-snapshot feed

NEXT_PUBLIC_DASHBOARD_SNAPSHOT=false built a dashboard that read its
tiles from two analytics slices, the widget layout, the mood analytics
and the thresholds instead of the one snapshot cell. It took a rebuilt
image to reach, and the snapshot has been the only path the published
image runs since it became the default. The flag, its readers, the
legacy reads in the dashboard hook and tile context, the slim and thick
merge helper, the build-time toggle notes in .env.production.example and
the self-hosting guide, and the tests that pinned the legacy wiring go.

docs/self-hosting/getting-started.md

@@ -195,26 +195,6 @@ bare Nginx — including the `X-Forwarded-For` chain shape that the
 rate-limiter trusts and the `TRUST_PROXY_HOPS` env var that pins how
 many hops to read back.
 
-## Build-time feature toggles
-
-A small number of toggles are `NEXT_PUBLIC_*` variables: the framework
-inlines them into the client bundle at **build time**, so the pre-built
-GHCR image cannot read them at runtime. Setting them in `.env` or the
-compose `environment:` block has no effect — they only take hold when
-you build your own image. They are intentionally absent from the
-compose runtime whitelist for that reason.
-
-| Variable                         | Default | Effect when set                                                                                                                                                                                                                                   |
-| -------------------------------- | ------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
-| `NEXT_PUBLIC_DASHBOARD_SNAPSHOT` | on      | The dashboard hydrates every above-the-fold tile from the single `GET /api/dashboard/snapshot` cell so the whole strip shares one completion moment. Build with `NEXT_PUBLIC_DASHBOARD_SNAPSHOT=false` to fall back to the legacy four-cell path. |
-
-To flip one, rebuild the image with the variable exported:
-
-```bash
-NEXT_PUBLIC_DASHBOARD_SNAPSHOT=false pnpm build
-# or pass it as a Dockerfile build ARG / Docker build --build-arg
-```
-
 ## 7. Where to go next
 
 | Next step                                                           | File                                  |