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/Cargo.toml

@@ -29,7 +29,7 @@ sysinfo = "0.32"
 image = "0.25"
 oxipng = "9"
 async_zip = { version = "0.0.17", features = ["tokio", "deflate"] }
-minijinja = { version = "2", features = ["json"] }
+include_dir = "0.7"
 
 [profile.release]
 opt-level = 3