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/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>