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.

frontend/src/watchEntry.contract.test.ts

@@ -99,6 +99,29 @@ function chain(seen: Map<string, string | null>, mod: string): string {
   return path.join(" → ");
 }
 
+describe("the paths a share link opens on", () => {
+  // The server's list: each of the places below that has to know them follows it.
+  const main = readFileSync(join(SRC, "..", "..", "backend", "app", "main.py"), "utf8");
+  const prefixes = [
+    ...(/SHARE_PREFIXES = \(([^)]*)\)/.exec(main)?.[1] ?? "").matchAll(/"(\w+)\/"/g),
+  ].map((m) => m[1]);
+
+  it("are read off the server", () => {
+    expect(prefixes).toEqual(["watch", "file", "folder"]);
+  });
+
+  it("are all kept out of search indexes", () => {
+    const robots = readFileSync(join(SRC, "..", "public", "robots.txt"), "utf8");
+    for (const p of prefixes) expect(robots).toContain(`Disallow: /${p}/`);
+  });
+
+  it("all boot the player alone in `vite dev` and all give up their token", () => {
+    const vite = readFileSync(join(SRC, "..", "vite.config.ts"), "utf8");
+    const page = readFileSync(join(SRC, "watch", "WatchPage.tsx"), "utf8");
+    for (const src of [vite, page]) expect(src).toContain(`(?:${prefixes.join("|")})`);
+  });
+});
+
 describe("the /watch entry", () => {
   it("loads nothing the app alone may load", () => {
     const seen = reach(join(SRC, "watchMain.tsx"));