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/store.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"database/sql"
+	"errors"
 	"fmt"
 	"strings"
 
@@ -28,6 +29,22 @@ type Bookmark struct {
 	UpdatedAt        int64    `json:"updated_at"`         // unix ms; see Upsert
 }
 
+// HasNewChapter reports whether the site has published past the read point.
+// A nil LatestChapterNum means nothing has been captured yet, which is not the
+// same as "nothing new".
+func (b Bookmark) HasNewChapter() bool {
+	return b.LatestChapterNum != nil && *b.LatestChapterNum > b.LastChapterNum
+}
+
+// ContinueURL is where the Continue button points: the chapter last read, or
+// the series page when no chapter URL was ever captured.
+func (b Bookmark) ContinueURL() string {
+	if b.LastChapterURL != "" {
+		return b.LastChapterURL
+	}
+	return b.SeriesURL
+}
+
 const schema = `
 CREATE TABLE IF NOT EXISTS bookmarks (
   key                TEXT PRIMARY KEY,
@@ -182,6 +199,21 @@ func (s *Store) List() ([]Bookmark, error) {
 	return out, rows.Err()
 }
 
+// Get returns one bookmark by key. A missing key is not an error: ok is false
+// and err is nil. UI mutations read-modify-write through this so they preserve
+// the fields they do not touch.
+func (s *Store) Get(key string) (Bookmark, bool, error) {
+	b, err := scanBookmark(s.db.QueryRow(
+		`SELECT `+bookmarkColumns+` FROM bookmarks WHERE key = ?`, key).Scan)
+	if errors.Is(err, sql.ErrNoRows) {
+		return Bookmark{}, false, nil
+	}
+	if err != nil {
+		return Bookmark{}, false, fmt.Errorf("get %q: %w", key, err)
+	}
+	return b, true, nil
+}
+
 // Upsert inserts or replaces a bookmark by key (last-write-wins) and returns
 // the row as actually stored.
 //