feat: replace HTML export with SvelteKit viewer

Replace the minijinja-based HTML export with a full SvelteKit static
viewer app. The new export produces a ZIP with:
- Pre-built viewer assets (index.html + JS/CSS bundle)
- data.json with all posts, comments, tags, and like counts
- Processed media: 400px thumbnails for grid, full images (2000px
  cap if >5MB), video thumbnails via ffmpeg

Remove minijinja dependency, add include_dir to embed viewer assets
at compile time. Update Dockerfile to copy static/ for builds.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/Dockerfile

@@ -11,6 +11,7 @@ RUN mkdir src && echo "fn main(){}" > src/main.rs && \
     rm -rf src
 
 COPY src ./src
+COPY static ./static
 RUN touch src/main.rs && cargo build --release
 
 # --- Runtime stage ---