Add comix.to and kagane.to support (#13)

Tracks read progress on comix.to and kagane.to alongside asura and demonic, in both the userscript and the backend.

Implements `docs/superpowers/plans/2026-08-03-comix-kagane-support.md`.

## Userscript

- `comix` adapter — `/title/<id>-<slug>`; only the id prefix is identity (the slug follows the title). No `og:image`, so the cover is matched by `alt`.
- `kagane` adapter — reader URLs are uuids with no chapter number, so it comes out of `og:title`; anchor scanning is structurally impossible, replaced by `latestChapterFromApi` against kagane's same-origin JSON API.
- `seriesId` threaded through `latestChapterFromAnchors` so comix can scope its scan to its own series and a recommendation strip cannot win the maximum.
- `@match` for both hosts, panel chips, v1.6.0.

## Backend

- `latestChapterFrom` cases: comix parses the SSR JSON state blob (`latestChapterUrl`, scoped to the series id); kagane parses API JSON (`chapter_no`).
- Poller allowlist extended; `Poller.BrowserFetch` with `fetcherFor(site)` routes kagane to a browser fetcher. Nil means kagane is not polled at all — never a fallback to the TLS fetcher, which would only ever retrieve a challenge page.
- `BrowserFetcher`: chromedp against a `headless-shell` sidecar. kagane sits behind a Cloudflare JS challenge that no TLS fingerprint clears, and the request is made inside the page rather than by replaying `cf_clearance`.
- `BROWSER_WS_URL` wiring, sidecar in both compose files (no `ports:`, dedicated non-external network), Dockerfile on `golang:1.26-alpine` — chromedp requires go 1.26.
- Web UI `--comix` / `--kagane` tokens in both colour branches.

## Notes for review

- `series_url` is client-supplied and a headless browser is a strong SSRF primitive, so kagane's host is pinned twice: in `fetchableSeriesURL` and again in `kaganeAPIURL`.
- Three chained defects found during verification made the browser path dead under Compose (sidecar flag collision, Chrome's Host-header DNS-rebinding check, the wrong chromedp option). Fixed; the compose comments record the wrong configurations too, so they don't get "simplified" back.
- `ALLOWED_ORIGINS` now includes both new origins. Without it every write from comix/kagane silently fails CORS preflight, parks in the retry queue, and drops at the cap.

## Verification

221 backend tests, 32 userscript tests, static `CGO_ENABLED=0` build, both compose configs.

Two gaps, both real:

1. The userscript on live pages via Violentmonkey needs a human browser profile — not run. Check: comix series page (title/cover, no chapter), comix chapter page (records the number; an *older* chapter must not regress it), comix SPA navigation without reload, kagane series page (og:image cover), kagane reader (number from `og:title`), both chips opening the right sites.
2. The kagane browser path has not completed end-to-end anywhere. Dial/navigate/fetch is confirmed, but Cloudflare 403'd headless-shell's Chrome on every attempt from the dev sandbox, and comix's poll-through-Docker was blocked by that environment's TLS interception. Both environment-dependent rather than branch defects — the first real deploy is the actual verification.

Reviewed-on: #13
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/web/templates/login.html

@@ -0,0 +1,33 @@
+{{define "login"}}
+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
+  <meta name="color-scheme" content="dark light">
+  <title>mangaBookmark</title>
+  <link rel="icon" href="/static/logo.svg" type="image/svg+xml">
+  <link rel="stylesheet" href="/static/style.css">
+  <link rel="preload" href="/static/fonts/instrument-serif-400-latin.woff2" as="font" type="font/woff2" crossorigin>
+</head>
+<body>
+  <main class="login-card">
+    <div>
+      <span class="eyebrow">Private library</span>
+      <h1 class="brand">{{template "mark" .}}<span>manga<em>Bookmark</em></span></h1>
+    </div>
+    <form method="post" action="/login">
+      <div>
+        <label for="password">Password</label>
+        <input id="password" name="password" type="password"
+               autocomplete="current-password" autofocus required>
+      </div>
+      {{/* The page reloads on a failed sign-in, so the message is present from
+           the start; role=alert is what gets it announced anyway. */}}
+      <p class="error" role="alert">{{.Error}}</p>
+      <button type="submit">Sign in</button>
+    </form>
+  </main>
+</body>
+</html>
+{{end}}