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).

backend/internal/web/templates/setup.html

@@ -12,6 +12,13 @@
     <a class="ghost" href="/install/manga-bookmark.user.js">Install Manga script</a>
     <a class="ghost" href="/install/novel-bookmark.user.js">Install Novels script</a>
   </p>
+  <p class="setup-copy">On mobile, Violentmonkey does not pick up the install
+    links — the script opens as text. Download the file instead, then add it
+    from Violentmonkey's own menu.</p>
+  <p class="setup-links">
+    <a class="ghost" href="/install/manga-bookmark.user.js?download=1">Download Manga script</a>
+    <a class="ghost" href="/install/novel-bookmark.user.js?download=1">Download Novels script</a>
+  </p>
   {{if .Rotated}}
   <p class="setup-warn" role="status">Credential rotated — the old one no
     longer works. Reinstall both scripts on every device now, or they will

backend/internal/web/web.go

@@ -547,6 +547,10 @@ func (h *Handler) uiDelete(w http.ResponseWriter, r *http.Request) {
 // derived credential substituted in. The credential is derived, not stored,
 // so installs work after any restart; the Reader never types or copies it —
 // clicking Install is the whole setup.
+//
+// ?download=1 forces a save instead. Mobile Violentmonkey (Chromium) does not
+// intercept navigation to a .user.js URL, so the Install link only renders the
+// source as text there; the Reader needs the file on disk to add it by hand.
 func (h *Handler) installUserscript(name string) http.HandlerFunc {
 	path := h.mangaUserscriptPath
 	if name == "novel-bookmark.user.js" {
@@ -559,6 +563,9 @@ func (h *Handler) installUserscript(name string) http.HandlerFunc {
 			http.Error(w, "internal error", http.StatusInternalServerError)
 			return
 		}
+		if r.URL.Query().Has("download") {
+			w.Header().Set("Content-Disposition", `attachment; filename="`+name+`"`)
+		}
 		userscript.Render(w, r, path, token.Token(h.tokenKey, discordID, epoch))
 	}
 }

backend/reader_credential_test.go

@@ -201,6 +201,27 @@ func TestInstallServesScriptWithCredential(t *testing.T) {
 		if loc := rr.Header().Get("Location"); loc != "" {
 			t.Fatalf("%s answered with a redirect, credential in Location %q", script, loc)
 		}
+		// The plain link must stay inline: Violentmonkey's updater polls the
+		// /u/ path and an attachment disposition there would break updates.
+		if cd := rr.Header().Get("Content-Disposition"); cd != "" {
+			t.Fatalf("%s served as %q, want inline", script, cd)
+		}
+
+		// ?download=1 is the mobile path: Violentmonkey on Chromium ignores a
+		// .user.js navigation, so the Reader saves the file and adds it by hand.
+		req = httptest.NewRequest(http.MethodGet, "/install/"+script+"?download=1", nil)
+		req.AddCookie(sessionCookie(t, st))
+		rr = httptest.NewRecorder()
+		srv.ServeHTTP(rr, req)
+		if rr.Code != http.StatusOK {
+			t.Fatalf("%s?download=1: status = %d, want 200", script, rr.Code)
+		}
+		if got, want := rr.Header().Get("Content-Disposition"), `attachment; filename="`+script+`"`; got != want {
+			t.Fatalf("%s?download=1: Content-Disposition = %q, want %q", script, got, want)
+		}
+		if !strings.Contains(rr.Body.String(), `API_TOKEN = "`+ownerCredential()+`"`) {
+			t.Fatalf("%s?download=1 does not carry the owner's credential", script)
+		}
 	}
 }
 
@@ -327,6 +348,8 @@ func TestIndexShowsSetupPanelWithoutCredential(t *testing.T) {
 	for _, want := range []string{
 		`href="/install/manga-bookmark.user.js"`,
 		`href="/install/novel-bookmark.user.js"`,
+		`href="/install/manga-bookmark.user.js?download=1"`,
+		`href="/install/novel-bookmark.user.js?download=1"`,
 		"Rotate credential",
 	} {
 		if !strings.Contains(body, want) {