feat: password-gated web UI on the same backend (#1)

Adds a password-gated browser UI for the bookmark list, served by the same Go
binary and container as the userscript API.

## What

- `GET /` — list page, or the login page when there is no session (200, no redirect).
- `POST /login`, `POST /logout` — stateless HMAC session cookie, 60-day Max-Age.
- `GET /ui/list?tab=all|fav`, `POST /ui/bookmarks/{key}/favorite`,
  `POST /ui/bookmarks/{key}/chapter`, `DELETE /ui/bookmarks/{key}` — htmx fragments.
- `GET /static/*` — embedded `style.css`, `htmx.min.js`, `filter.js`.

Mobile-first dark CSS, 2–3 column grid at ≥900px, "Continue reading" strip of the
five most recent series, NEW badge, client-side title search, no build step.

## Stack

Go `html/template` + htmx 2.0.4 (vendored, 50 KB) + plain CSS. No npm, no bundler.
Templates and assets are `go:embed`-ed, so `CGO_ENABLED=0` and the distroless
image still hold.

## Auth

`WEB_PASSWORD` gates the UI; unset means the web routes are never registered and
`/` returns 404. Session cookie is `HttpOnly`, `SameSite=Lax`, `Secure` when the
request is HTTPS. The signing key derives from `API_TOKEN` + `WEB_PASSWORD`, so
rotating either logs every browser out. Login is rate-limited to 10 failures per
20 minutes per client IP, keyed on the **rightmost** `X-Forwarded-For` entry
(Traefik appends the observed peer, so the leftmost is client-spoofable). CGNAT
lockout is a known, accepted limitation — the window self-heals.

## Invariants preserved

- A session cookie never authenticates `/bookmarks*`. That API stays JSON +
  bearer token, unchanged, as does the userscript.
- `Store.Upsert` is byte-for-byte unmodified. Every UI write goes
  read-modify-write through the new `Store.Get`, so the conditional-`updated_at`
  rule (favouriting must not reorder the list, a chapter override must) lives in
  exactly one function.

## Deployment

`docker-compose.prod.yml` gains a second Traefik router on `MANGA_WEB_HOST`
pointing at the same service — one container, one certificate resolver, no second
service. Both `MANGA_API_HOST` and `MANGA_WEB_HOST` are required (`:?`), with no
example fallback in `.env.example`: a placeholder there would make Traefik
silently publish the UI on a domain you do not own. Needs a DNS A/AAAA record for
`manga.<domain>`. See `DEPLOY.md` §1b.

## Docs

- Design: `docs/superpowers/specs/2026-07-25-web-ui-design.md`
- Plan: `plans/2026-07-25-web-ui-implementation-plan.md`

## Verification

`gofmt` clean, `go vet`, `go test -race ./...`, `CGO_ENABLED=0 go build`, a real
`docker build` + curl smoke test, and a Playwright pass covering login
reject/accept, favourite-without-reorder, chapter edit, delete-with-confirm,
search, tab switch + back button, 390px with no horizontal overflow, and zero JS
console errors.

Reviewed-on: #1
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/templates/app.html

@@ -0,0 +1,53 @@
+{{define "app"}}
+<!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="stylesheet" href="/static/style.css">
+  <script src="/static/htmx.min.js" defer></script>
+  <script src="/static/filter.js" defer></script>
+</head>
+<body>
+  <header class="topbar">
+    <h1>mangaBookmark</h1>
+    <form method="post" action="/logout">
+      <button type="submit" class="ghost">Log out</button>
+    </form>
+  </header>
+
+  <input id="search" class="search" type="search" placeholder="Search titles…"
+         autocomplete="off" aria-label="Search titles">
+
+  <nav class="tabs" role="tablist">
+    <a role="tab" href="/?tab=all" class="{{if eq .Tab "all"}}active{{end}}"
+       hx-get="/ui/list?tab=all" hx-target="#list" hx-swap="innerHTML"
+       hx-push-url="/?tab=all" hx-on::after-request="setActiveTab(this)">All</a>
+    <a role="tab" href="/?tab=fav" class="{{if eq .Tab "fav"}}active{{end}}"
+       hx-get="/ui/list?tab=fav" hx-target="#list" hx-swap="innerHTML"
+       hx-push-url="/?tab=fav" hx-on::after-request="setActiveTab(this)">Favourites</a>
+  </nav>
+
+  {{if .Recent}}
+  <section class="recent">
+    <h2>Continue reading</h2>
+    <div class="recent-strip">
+      {{range .Recent}}
+      <a class="recent-card" href="{{.ContinueURL}}" target="_blank" rel="noopener noreferrer">
+        {{if .Cover}}<img src="{{.Cover}}" alt="" loading="lazy">{{end}}
+        <span class="recent-title">{{.Title}}</span>
+        <span class="recent-chapter">Ch {{.LastChapter}}</span>
+      </a>
+      {{end}}
+    </div>
+  </section>
+  {{end}}
+
+  <main id="list" class="list">
+    {{template "list" .}}
+  </main>
+</body>
+</html>
+{{end}}