Cinder pass across /admin, the login gate, and the library's a11y floor (#177)

One commit (`af07314`), three strands of browser-UI work against one design system. `docs/design-system.md` was updated to match the CSS, not the reverse.

## Library (Reader-facing)

Findings came out of a two-axis design review of the library surface; the fixes are the P1/P2 set plus the cheap P3s.

- **`.chrome` sticks at `top: 0`.** Search and the tab row were unreachable three screens into a 300-item library — exactly where they earn their keep. Everything above them (`.topbar`, `.keyrow`, `.recent`) still scrolls away on purpose: another 150px of permanent chrome on an 844px phone costs more than re-scrolling for an icon reminder.
- **One `:focus-visible` ring** (`2px solid var(--paper)`, offset 2px) on the nine controls that defined none and fell back to the UA blue — a colour tuned for neither branch of this palette. `.searchbar` keeps its `:focus-within` border recolour as a resting cue but no longer stands in for the ring.
- **Mono labels lift 10px → 11px** everywhere (nine rules). PRODUCT.md names night reading and glare as the usage scene; 10px small-caps was the one place taste overrode the brief. 11px is now a documented floor.
- **A card in flight past 2s says `Saving…` and carries `aria-busy`.** htmx sets neither, so the wait — up to its own 15s timeout, and this app is used on a phone in dead zones — was silent in both the visual and the assistive channel. Deliberately `--mute`, not `--ember`: ember means "new chapter" and nothing else.
- **Titles clamp at 3 lines**; `.is-new .title` takes `width: fit-content`, or `-webkit-box` stretches the ember underline past the text it is supposed to be sized to.
- `.libswitch a` reaches a real 44px under `(pointer: coarse)` — padding plus an 11px line landed at 43.

## /admin

- Overview routes into Lanes when a lane is unhealthy, prefixes each figure with its column word on the phone layout that drops the `thead`, labels state cells for a screen reader, and has an empty state where the sites table previously assumed rows.
- The admin shell picks up the library's chrome: htmx 15s timeout, the shared `#notice` slot, `#sr-announce`, `filter.js`.
- `admin_render_test.go` and `card_render_test.go` render the templates directly, so markup regressions in either surface fail without a browser.

## Login

`DISCORD_GUILD_NAME` (optional) names the community on the login screen and in the refusal message, so a stranger knows which Discord to ask for an invite. Unset degrades to a generic label. Neither form names the numeric guild id — that was never actionable, and the gate still reveals nothing about whether a given guild exists.

## Handlers

`maxChapterNum` (9999) now bounds **both** typed-chapter paths. `uiChapter` and `adminSeriesCorrectLatest` each parsed a `float64` with no ceiling, so a hand-rolled POST stored `1e308` and every later reader of that row — the poller's `HasNewChapter` comparison, the display string — inherited it. Matches the `max` on the card's chapter input. The API PUT path is deliberately untouched: it carries the userscripts' own scraped numbers, not typed input.

## Verification

- `cd backend && go test ./...` green (Docker-backed `pgtest`). `TestChapterOverrideRejectsBadInput` gained `"10000"` and `"1e5"` — both parse fine as `float64`, so they only fail if the bound exists.
- Visual: 390×844 dark + light, 1000px and 1440px (`zoom: 1.2`) desktop, against the real templates + real CSS. Measured `chromeTop = 0` at `scrollY 950`, `2px solid rgb(242,236,229)` rings, `content: "Saving…"` at `opacity: 1` after 2.4s, `aria-busy` `true` during / cleared after, `libswitchH = 44` in a `hasTouch` context, no horizontal overflow at either width.
- `detect.mjs` on `templates/`: `[]`, exit 0.

## Note on shape

The three strands landed as one commit because `admin_series.go` and `style.css` each carry hunks from more than one of them; splitting cleanly would have needed hunk-level surgery. Say the word if you want it split before merge.

Reviewed-on: #177
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/web/templates/chrome.html

@@ -14,8 +14,9 @@
     <a class="recent-card {{if .HasNewChapter}}is-new{{end}}" href="{{.ContinueURL}}"
        target="_blank" rel="noopener noreferrer">
       <span class="recent-cover">
-        {{if .Cover}}<img src="{{.Cover}}" alt="" loading="lazy">
-        {{else}}<span class="monogram" aria-hidden="true">{{.Initial}}</span>{{end}}
+        {{/* Monogram always present under the image; see card.html. */}}
+        {{if .Cover}}<img src="{{.Cover}}" alt="" loading="lazy" onerror="this.remove()">{{end}}
+        <span class="monogram" aria-hidden="true">{{.Initial}}</span>
         {{if .HasNewChapter}}<span class="foot-rule"></span>
         {{else if .Favorite}}<span class="foot-rule brass"></span>{{end}}
       </span>
@@ -34,11 +35,11 @@
 <div class="keyrow" id="keyrow" aria-label="Action key"{{if .OOB}} hx-swap-oob="true"{{end}}>
   <span class="pair"><svg viewBox="0 0 24 24" aria-hidden="true"><use href="#i-play"/></svg><span>Read</span></span>
   <span class="pair brass"><svg viewBox="0 0 24 24" aria-hidden="true"><use href="#i-star"/></svg><span>Fav</span></span>
-  <span class="pair"><svg viewBox="0 0 24 24" aria-hidden="true"><use href="#i-pencil"/></svg><span>Chapter</span></span>
+  <span class="pair clay"><svg viewBox="0 0 24 24" aria-hidden="true"><use href="#i-pencil"/></svg><span>Chapter</span></span>
   {{if eq .Tab "archived"}}
-  <span class="pair"><svg viewBox="0 0 24 24" aria-hidden="true"><use href="#i-undo"/></svg><span>Restore</span></span>
+  <span class="pair slate"><svg viewBox="0 0 24 24" aria-hidden="true"><use href="#i-undo"/></svg><span>Restore</span></span>
   {{else}}
-  <span class="pair"><svg viewBox="0 0 24 24" aria-hidden="true"><use href="#i-box"/></svg><span>Archive</span></span>
+  <span class="pair slate"><svg viewBox="0 0 24 24" aria-hidden="true"><use href="#i-box"/></svg><span>Archive</span></span>
   {{end}}
   <span class="pair trash"><svg viewBox="0 0 24 24" aria-hidden="true"><use href="#i-trash"/></svg><span>Delete</span></span>
 </div>