docs: track product context and UI critique snapshots

PRODUCT.md records the durable constraints the UI work keeps having to
rediscover: dark-first is binding (night reading), mobile is the target
rather than the starting breakpoint, and updated_at ordering is a
correctness rule the design must not decorate over.

The two critique snapshots are the scored before/after (21/40, then
26/40) the card fixes were driven from — worth keeping as the record of
which problems were already known, and which ones the fixes introduced.

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

PRODUCT.md

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+# Product
+
+<!-- impeccable:product-schema 1 -->
+
+## Platform
+
+web
+
+## Users
+
+Single user (self-hosted, no accounts, no multi-user planned). Reads manga on **asurascans.com** and **demonicscans.org** primarily via Bromite on mobile, also checks/updates from a desktop browser. The web UI is the cross-device view into progress captured by the userscript while reading.
+
+## Product Purpose
+
+Tracks read-progress ("last chapter read") per manga series across two otherwise-unrelated manga sites that each have their own separate `localStorage`. A Go backend unifies bookmarks into one store; the web UI is a password-gated browser view of that store for reviewing, favouriting, correcting, or removing bookmarks, and jumping back into a series to continue reading. A background poller also refreshes each series' latest-published-chapter so the list can flag "NEW" without the user visiting the site.
+
+## Positioning
+
+Not a public reading tracker or social app — a private, self-hosted sync layer purpose-built for two specific scraped sites, with no server-side account system (single bearer token + one password-gated session).
+
+## Operating Context
+
+- Primary reading device: Bromite (mobile Chromium), where a userscript captures progress automatically.
+- Web UI is a secondary surface: checking list state, correcting a wrong chapter number, removing dead bookmarks, jumping to "continue reading."
+- Manga cover art and titles come from the source sites' `og:image`/`og:title` — real content, not placeholders.
+- List order is driven by `updated_at`, which moves only on real reading progress (not favouriting, not a newly detected chapter) — a UI constraint the redesign must not break.
+
+## Capabilities and Constraints
+
+- Two tabs: All / Favourites. Search-filter by title (client-side, `filter.js`).
+- Card actions: continue (opens source site), toggle favourite, manual chapter override, delete (with confirm).
+- "Continue reading" horizontal strip for recently-progressed series.
+- htmx-driven partial updates (card re-render on favourite/chapter/delete), no client-side framework/build step — templates are Go `html/template`, `go:embed`-ed.
+- Mobile-first is a hard functional constraint (primary device is a phone), not just a starting breakpoint.
+
+## Brand Commitments
+
+- Name: **mangaBookmark**.
+- **Dark-first is binding**: current dark-by-default / light-follows-system-preference behavior must be preserved as a design constraint, not just a starting default, because reading happens at night.
+
+## Evidence on Hand
+
+- Live templates/CSS at `backend/templates/*.html`, `backend/static/style.css` — current implemented UI, functional but not yet treated as an intentional design system.
+- No logo, screenshots, or marketing copy exist; none should be fabricated.
+
+## Product Principles
+
+- Dark-first, night-reading-optimized — never regress to a light-default or high-glare surface.
+- Mobile is the primary target; desktop is an enhancement, not the design center.
+- Progress data integrity over visual flourish: `updated_at`/list-ordering behavior is a correctness constraint the UI must respect, not decorate over.
+- No accounts, no multi-tenant chrome — the whole product is for one reader.
+- Prefer native platform affordances (system dark/light, native touch targets) over custom widgetry — this is a lean self-hosted tool, not a product to demo.
+
+## Accessibility & Inclusion
+
+Explicit personal requirement: optimized for **low-light/night reading** — minimize glare and eye strain (true dark surfaces, restrained brightness/contrast on accents, no jarring pure-white flashes), beyond generic touch-target/contrast compliance.