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/admin.html

@@ -7,12 +7,17 @@
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
   <meta name="color-scheme" content="dark light">
+  {{/* Same request timeout as the library shell: the Lanes fragment refreshes
+       itself every 30s, so an untimed hung request stacks. */}}
+  <meta name="htmx-config" content='{"timeout":15000}'>
   <title>BookmarkManager — Admin</title>
   <link rel="icon" href="/static/logo.svg" type="image/svg+xml">
   <link rel="stylesheet" href="/static/style.css">
   <link rel="stylesheet" href="/static/admin.css">
   <link rel="preload" href="/static/fonts/instrument-serif-400-latin.woff2" as="font" type="font/woff2" crossorigin>
+  <link rel="preload" href="/static/fonts/dm-sans-var-latin.woff2" as="font" type="font/woff2" crossorigin>
   <script src="/static/htmx.min.js" defer></script>
+  <script src="/static/filter.js" defer></script>
 </head>
 <body>
 <div class="sheet admin-sheet">
@@ -33,9 +38,14 @@
     <a href="/admin/series" class="{{if or (eq .Page "series") (eq .Page "series-detail")}}active{{end}}" {{if or (eq .Page "series") (eq .Page "series-detail")}}aria-current="page"{{end}}>Series</a>
   </nav>
 
+  {{/* Where a failure with no specific slot gets reported — same slot the
+       reader library uses under its chrome. filter.js fills it. */}}
+  <p class="notice" id="notice" role="status" hidden></p>
+<div id="sr-announce" class="sr-only" role="status" aria-live="polite" aria-atomic="true"></div>
+
   <main class="page admin-page">
     {{if eq .Page "lanes"}}
-    <div aria-live="polite">{{template "lanes" .Lanes}}</div>
+    {{template "lanes" .Lanes}}
     {{else if eq .Page "readers"}}
     {{template "readers" .}}
     {{else if eq .Page "series"}}