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/main.go

@@ -18,6 +18,8 @@ type Config struct {
 	AllowedOrigins []string
 	DBPath         string
 	Port           string
+	// WebPassword gates the browser UI. Empty disables the web routes entirely.
+	WebPassword string
 }
 
 func envOr(key, def string) string {
@@ -29,9 +31,10 @@ func envOr(key, def string) string {
 
 func loadConfig() Config {
 	c := Config{
-		Token:  os.Getenv("API_TOKEN"),
-		DBPath: envOr("DB_PATH", "/data/bookmarks.db"),
-		Port:   envOr("PORT", "8080"),
+		Token:       os.Getenv("API_TOKEN"),
+		DBPath:      envOr("DB_PATH", "/data/bookmarks.db"),
+		Port:        envOr("PORT", "8080"),
+		WebPassword: os.Getenv("WEB_PASSWORD"),
 	}
 	for _, o := range strings.Split(os.Getenv("ALLOWED_ORIGINS"), ",") {
 		if o = strings.TrimSpace(o); o != "" {
@@ -58,6 +61,17 @@ func newRouter(store *Store, cfg Config) http.Handler {
 	mux.Handle("/bookmarks", auth)
 	mux.Handle("/bookmarks/", auth)
 
+	// The browser UI is registered only when a password is configured, so a
+	// deployment that forgets WEB_PASSWORD exposes nothing rather than
+	// exposing an unprotected list.
+	if cfg.WebPassword != "" {
+		web, err := newWebHandler(store, cfg)
+		if err != nil {
+			log.Fatalf("web handler: %v", err)
+		}
+		web.register(mux)
+	}
+
 	return withCORS(cfg.AllowedOrigins, mux)
 }