feat: activity sidecar edits via bincio edit --serve

- bincio/render/merge.py: parse sidecar .md files (YAML frontmatter +
     markdown body), produce data/_merged/ with symlinks for unmodified
     activities and real merged files for overridden ones; filters private
     activities from index.json; sorts highlighted activities first.
     Keeps extracted data pristine — re-running extract never clobbers edits.

   - bincio/edit/: FastAPI edit server (port 4041) with embedded HTML/JS
     edit UI; GET/POST /api/activity/{id} reads/writes sidecars; multipart
     image upload to edits/images/{id}/; DELETE for image cleanup.

   - bincio render now calls merge_all() before build/serve and symlinks
     public/data → data/_merged/ instead of data/ directly.

   - ActivityDetail.svelte: edit button (links to edit server) when
     PUBLIC_EDIT_URL env var is set; respects custom.hide_stats to suppress
     stat panels; description supports whitespace-preserving rendering.

   - 15 unit tests covering parse_sidecar, apply_sidecar, and merge_all.

pyproject.toml

@@ -32,6 +32,11 @@ dependencies = [
 classifier = [
     "scikit-learn>=1.5",
 ]
+edit = [
+    "fastapi>=0.110",
+    "uvicorn[standard]>=0.29",
+    "python-multipart>=0.0.9",
+]
 dev = [
     "pytest>=9.0",
     "pytest-cov>=5.0",