feat(share): a link's address says what it opens

A video stays on /watch/, anything else is given out on /file/ and a folder on /folder/. The token
is the whole key, so every path opens every link and the links already sent keep working. The new
paths are kept out of search indexes like /watch/, boot the player alone, and the unfurl card calls a
file a file.

CLAUDE.md

@@ -74,7 +74,7 @@ only along a declared edge and writes them only through that unit's service; cor
 cannot (an ORM write on another unit's loaded row, SQL in `text()`, a commit by `begin()`) is review's to catch, listed in
 [`backend/app/README.md`](backend/app/README.md) with the conventions.
 Frontend, by folder under `src/`: `main.tsx` provider tree → `shell/` (`App`, header, nav rail, palette,
-boot; a shared `/watch` link boots from its own entry, `watch.html` → `watchMain.tsx` → `watch/`, held
+boot; a share link (`/watch`, `/file`, `/folder`) boots from its own entry, `watch.html` → `watchMain.tsx` → `watch/`, held
 app-free by a contract test) → **`modules/<page>/`** (one folder per page: the page, its rail, its store, its tests,
 and its `module.ts` definition — label, icon, gate, the lazy page and rail, its store — collected in nav order by
 `features/navigation/moduleRegistry`, which also mounts every store at the root; a new page is its folder, its id in