feat(web): offer the userscripts as a download for mobile

Violentmonkey on mobile Chromium does not intercept navigation to a
.user.js URL, so the Install link renders the script as text and there is
no way to get it installed. Adding ?download=1 sets Content-Disposition:
attachment on the same session-gated endpoint, so the Reader saves the
file and adds it from Violentmonkey's own menu.

The plain link stays inline on purpose: the updater polls the /u/ path and
an attachment disposition there would break auto-update. The test asserts
both halves.

Refs #26

backend/AGENTS.md

@@ -127,6 +127,8 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
 - **Web UI also owns:** session-gated `GET /install/{manga,novel}-bookmark.user.js`
   (renders the bindmounted script with the acting Reader's derived credential
   substituted in — the credential never appears in page markup, the address
-  bar, or a redirect) and `POST /rotate-token` (atomic epoch bump + hash
+  bar, or a redirect; `?download=1` adds `Content-Disposition: attachment` for
+  mobile Violentmonkey, which ignores a `.user.js` navigation) and
+  `POST /rotate-token` (atomic epoch bump + hash
   rewrite; invalidates every installed copy, so the panel warns to reinstall
   on all devices).