Cinder pass across /admin, the login gate, and the library's a11y floor

Uncommitted work from three design runs on this branch, against one design
system: docs/design-system.md is updated to match the CSS, not the reverse.

Library (Reader-facing):
- .chrome sticks at top: 0. Search and the tab row were unreachable three
  screens into a 300-item library, which is exactly where they earn their
  keep; everything above them still scrolls away on purpose.
- One :focus-visible ring (2px --paper) on the nine controls that defined
  none and fell back to the UA blue. .searchbar keeps its border recolour as
  a resting cue but no longer stands in for a ring.
- Mono labels lift 10px -> 11px everywhere. The brief names night reading and
  glare as the usage scene; 10px small-caps was where taste overrode it.
- A card in flight past 2s says "Saving..." and carries aria-busy. htmx sets
  neither, so the wait up to its 15s timeout was silent in both channels.
- Titles clamp at 3 lines; .is-new .title takes width: fit-content, or
  -webkit-box stretches the ember underline past the text it sizes to.

/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
  assumed rows.
- The admin shell picks up the library's chrome: htmx 15s timeout, the shared
  #notice slot, #sr-announce, filter.js. admin.css follows the same pass.
- 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 guild id.

Handlers:
- maxChapterNum (9999) 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 inherited
  it. Matches the max on the card's chapter input.

go test ./... green.

backend/internal/web/templates/card.html

@@ -6,11 +6,17 @@
   <div class="row">
     <a class="cover" href="{{.ContinueURL}}" target="_blank" rel="noopener noreferrer"
        tabindex="-1" aria-hidden="true">
-      {{if .Cover}}<img src="{{.Cover}}" alt="" loading="lazy">
       {{/* aria-hidden on the cover link is not enough — Chromium still exposes
            the letter because the link is programmatically focusable — so the
-           monogram carries its own, same as the recent strip's. */}}
-      {{else}}<span class="monogram" aria-hidden="true">{{.Initial}}</span>{{end}}
+           monogram carries its own, same as the recent strip's.
+
+           The monogram is always in the DOM under the image, not only when no
+           cover was acquired: a stored cover that 404s or a phone that drops
+           the request otherwise leaves the browser's broken-image glyph in a
+           93px slot. onerror removes the image and the letter is already
+           there. */}}
+      {{if .Cover}}<img src="{{.Cover}}" alt="" loading="lazy" onerror="this.remove()">{{end}}
+      <span class="monogram" aria-hidden="true">{{.Initial}}</span>
       {{if and (eq .Status "reading") .HasNewChapter}}<span class="foot-rule"></span>
       {{else if .Favorite}}<span class="foot-rule brass"></span>{{end}}
     </a>
@@ -46,13 +52,14 @@
       </a>
       <button class="fav{{if .Favorite}} on{{end}}"
               title="{{if .Favorite}}Remove from favourites{{else}}Add to favourites{{end}}"
-              aria-label="Toggle favourite"
+              aria-label="{{if .Favorite}}Remove from favourites{{else}}Add to favourites{{end}}"
+              aria-pressed="{{if .Favorite}}true{{else}}false{{end}}"
               hx-post="/ui/bookmarks/{{.Key}}/favorite"
               hx-target="[id='card-{{.Key}}']" hx-swap="outerHTML"
               hx-indicator="[id='card-{{.Key}}']" hx-disabled-elt="this">
         <svg viewBox="0 0 24 24" aria-hidden="true"><use href="#i-star{{if .Favorite}}-on{{end}}"/></svg>
       </button>
-      <button class="pencil" title="Set chapter" aria-label="Set chapter"
+      <button class="pencil" title="Set chapter" aria-label="Set chapter" aria-expanded="false" aria-controls="chapter-form-{{.Key}}"
               onclick="toggleChapterForm('{{.Key}}')">
         <svg viewBox="0 0 24 24" aria-hidden="true"><use href="#i-pencil"/></svg>
       </button>
@@ -88,8 +95,11 @@
          label names the field and the latest sits after it as context. */}}
     <label class="hint" for="chapter-{{.Key}}">Chapter you're on</label>
     <div class="field">
-      {{/* max is a fat-finger guard, not a real ceiling — no series is near it. */}}
-      <input id="chapter-{{.Key}}" name="chapter" type="number" step="0.1" min="0" max="9999"
+      {{/* max is a fat-finger guard, not a real ceiling — no series is near it.
+           step is "any" because the server accepts any float and the field is
+           pre-filled from it: under step=0.1 a series read to 1200.25 opened a
+           form that refused its own value. */}}
+      <input id="chapter-{{.Key}}" name="chapter" type="number" step="any" min="0" max="9999"
              value="{{.LastChapterNum}}" required>
       <button type="submit">Save</button>
     </div>