Build the SvelteKit frontend: calm home with mood modes

- New frontend/ SvelteKit static SPA (Svelte 5), served by FastAPI from
  frontend/build (falls back to the legacy page if unbuilt).
- Calm design system: cream/sage palette, serif headlines, generous space,
  no urgency colors, gentle motion (respects prefers-reduced-motion).
- Home screen: mood-mode nav (Today/Wonder/People Helping/Solutions/Light
  Only/Grounded), the daily brief as a hero + remaining four, browsable mood
  lanes, an explicit calm end-state, inline Not today / Less like this / Hide
  affordances, and device-local Calm Filters mirroring goodnews/filters.py.
- Backend: moods.py + GET /api/moods (single source of truth for the modes);
  FilterPrefs gains max_cortisol/max_ragebait ceilings (for Light Only).
- Push categorical filters (include/mute topics+flavors, ceilings) into SQL in
  queries.feed so low-ranked-but-matching items (e.g. discovery for Wonder)
  are not truncated by ranking; only avoid-terms stay a Python pass.
- PWA manifest + icon (installable; offline deferred per plan).
- Multi-stage Dockerfile builds the site then serves it from the API.
- Tests: queries.feed categorical filters (63 total). README updated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -101,6 +101,7 @@ Endpoints:
 - `GET /` — the static site (daily five + topic/flavor browsing)
 - `GET /healthz` — liveness + scored-article count
 - `GET /api/categories` — the topic/flavor taxonomy
+- `GET /api/moods` — mood modes (the humane front door: Today, Wonder, People Helping, Solutions, Light Only, Grounded)
 - `GET /api/category-counts` — article counts per topic/flavor
 - `GET /api/feed?topic=&flavor=&limit=&offset=` — ranked, filtered articles
 - `GET /api/brief?date=&limit=` — a daily brief (latest if no date)
@@ -112,6 +113,22 @@ Endpoints:
 The ingestion CLI stays pure-stdlib; only the `web` extra pulls in FastAPI/uvicorn,
 so the two halves can be deployed and upgraded independently.
 
+### Frontend
+
+The site is a SvelteKit static SPA in `frontend/` (calm editorial design, mood-mode
+navigation, the daily brief as a hero, browsable lanes, inline Calm Filters, PWA
+manifest). It consumes the JSON API above, so the website and a future companion
+app share one contract. Build it once and FastAPI serves the output:
+
+```bash
+cd frontend && npm install && npm run build   # -> frontend/build
+cd .. && python3 -m goodnews serve             # serves frontend/build at /
+```
+
+If `frontend/build` is absent, the server falls back to the legacy single-page
+harness in `goodnews/static/`. The Docker image builds the frontend automatically
+(multi-stage), so deployment is just `docker build`.
+
 ## Calm Filters
 
 Personal, device-local controls so a reader can stay informed without subjects