feat: wire headless-shell sidecar for challenge-gated polling

Also bumps backend/Dockerfile's build stage to golang:1.26-alpine —
chromedp v0.16.0 and cdproto both require go 1.26, and the pinned
1.24-alpine base no longer builds the module.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -1,8 +1,9 @@
 # Manga Bookmark
 
-Track manga read-progress on **asurascans.com** (a.k.a. asuracomic.net) and
-**demonicscans.org** from a phone (Bromite / mobile Chromium), synced to a
-self-hosted Go backend so bookmarks unify across both sites and all devices.
+Track manga read-progress on **asurascans.com** (a.k.a. asuracomic.net),
+**demonicscans.org**, **comix.to**, and **kagane.to** from a phone (Bromite /
+mobile Chromium), synced to a self-hosted Go backend so bookmarks unify across
+all four sites and all devices.
 
 Two parts: