feat(standalone): browser-local (IndexedDB) virtual projects

Loading a folder now imports a COPY into a browser-local IndexedDB project with
its own /p/:slug URL (original disk files untouched): edits persist to IDB across
visits, and the project exports via Download .zip or per-file. Gated by
VITE_LOCAL_PROJECTS=idb (on for the demo; off keeps the File System Access
write-back flow), so it's configurable per deployment.

Modular, swappable project sources behind the shared ProjectSource interface,
each self-describing via a SourceDescriptor whose kind is shown verbatim in the
UI — "Local (this browser)", "Remote · read-only", "Remote · editable" — on the
home page, the project view, and inside the editor, so the user always knows
whether/how Save persists:
  - remote      → REST backend (remote-rw)
  - static      → CDN gallery (remote-ro), saves download
  - local (new) → idbProjectStore, writable IDB store
A composite layers the local store over the configured remote/gallery source,
routing per slug so imported/saved projects and the gallery share one namespace.

New, dependency-free (matching sync-client's raw-IDB ethos):
  - lib/idb-project-store.ts  raw IndexedDB store + create/delete/rename/export
  - lib/zip.ts                store-only ZIP writer (verified via system unzip)
  - lib/import-folder.ts      FSA/webkitdirectory folder → in-memory bytes
  - lib/project-source-shared.ts  source descriptors + deterministic uuid
  - components/SourceChip, components/LocalProjectsSection
Home lists local projects (open/export/rename/delete). Verified: typecheck,
project-source tests, prod build, and IDB key-range project isolation in-browser
(prefix-colliding slugs don't leak).

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

scripts/deploy/build-demo.mjs

@@ -71,6 +71,9 @@ function main() {
     // Read-only example gallery, saves download to local.
     VITE_PROJECT_SOURCE: "static",
     VITE_PROJECT_MANIFEST_URL: `${a.cdn}/content/${a.tag}/manifest.json`,
+    // Loaded folders import into a browser-local (IndexedDB) project — editable,
+    // persistent, exported via Download .zip — layered over the gallery.
+    VITE_LOCAL_PROJECTS: "idb",
     // Built-in offline symbols (no backend); cross-tab collab only.
     VITE_LIBS_SOURCE: "static",
     VITE_YJS_PROVIDER: "broadcastchannel",