docs: realign blueprint with shipped state + add feature/journey/ideas docs

- PROJECT.md, README.md, TEST_GUIDE.md: status line refreshed; rate-limiter
  doc-vs-code drift fixed; HTML export section rewritten for the SvelteKit-
  static viewer; SSE event names + new events documented; config seed block
  extended with planned toggles + privacy_note; decision log entries added.
- docs/CONCEPT_HTML_VIEWER.md, docs/CONCEPT_MOBILE_UI.md: banner the design
  intent as shipped; point at the source-of-truth code paths.
- docs/CONCEPT_DIASHOW.md: planned-then-shipped design for the live diashow
  (two-queue policy, pluggable transitions, data-mode aware).
- docs/FEATURES.md: capability matrix by role (Guest / Host / Admin) plus
  prose per area (auth, posting, feed, moderation, admin, export, gestures,
  data mode, quotas, privacy note, extensibility).
- docs/USER_JOURNEYS.md: step-by-step flows for every supported scenario,
  including PIN reset by host, data mode, privacy note, gestures, and the
  admin toggles.
- docs/IDEAS.md: speculative extensions (global diashow, reactions,
  multi-tenancy, animation pack, etc.) — explicitly out of v0.16 scope.
- backend/migrations/README.md, frontend/src/lib/README.md: codify the
  "never edit a shipped migration" rule and the lib/ conventions
  (one store per concern, gestures via actions, sheets via ContextSheet,
  transitions as drop-in components).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/CONCEPT_HTML_VIEWER.md

@@ -1,5 +1,15 @@
 # HTML Viewer Export Concept
 
+> **Status: IMPLEMENTED.** Viewer source: [frontend/export-viewer/](../frontend/export-viewer/).
+> Pre-built output committed to [backend/static/export-viewer/](../backend/static/export-viewer/).
+> Backend export pipeline: [backend/src/services/export.rs](../backend/src/services/export.rs).
+>
+> Outstanding follow-ups:
+> - The export-viewer's `tailwind.config.js` does not yet extend the main app's config. Visual
+>   drift risk — see "Shared Tailwind Config" section below.
+> - Service-worker (offline PWA caching) is still "Future" — fine for v1 since the ZIP is
+>   already fully offline by virtue of relative paths.
+
 ## Overview
 
 The HTML Viewer export produces a **self-contained offline ZIP** that is a read-only clone