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/app.html

@@ -5,6 +5,11 @@
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
   <meta name="color-scheme" content="dark light">
+  {{/* htmx defaults to no timeout, so a phone that walks into a dead zone
+       leaves the XHR open forever and the card it came from keeps its busy bar
+       and its disabled actions until a reload. 15s is well past any response
+       this deployment produces. */}}
+  <meta name="htmx-config" content='{"timeout":15000}'>
   <title>BookmarkManager</title>
   <link rel="icon" href="/static/logo.svg" type="image/svg+xml">
   <link rel="stylesheet" href="/static/style.css">
@@ -53,34 +58,39 @@
     <a href="{{.PageURL "all"}}" class="{{if eq .Tab "all"}}active{{end}}"
        {{if eq .Tab "all"}}aria-current="page"{{end}}
        hx-get="{{.ListURL "all"}}" hx-target="#list" hx-swap="innerHTML"
-       hx-push-url="{{.PageURL "all"}}" hx-on::after-request="setActiveTab(this)">All</a>
+       hx-push-url="{{.PageURL "all"}}" hx-on::after-request="if (event.detail.successful) setActiveTab(this)">All</a>
     {{/* The one bucket novels do not have: without a poller-fed "what is out
          that I have not read", the tab would only ever restate All. */}}
     {{if eq .Lib "manga"}}
     <a href="{{.PageURL "new"}}" class="tab-new {{if eq .Tab "new"}}active{{end}}"
        {{if eq .Tab "new"}}aria-current="page"{{end}}
        hx-get="{{.ListURL "new"}}" hx-target="#list" hx-swap="innerHTML"
-       hx-push-url="{{.PageURL "new"}}" hx-on::after-request="setActiveTab(this)">Updated
+       hx-push-url="{{.PageURL "new"}}" hx-on::after-request="if (event.detail.successful) setActiveTab(this)">Updated
       {{template "newcount" .}}</a>
     {{end}}
     <a href="{{.PageURL "fav"}}" class="{{if eq .Tab "fav"}}active{{end}}"
        {{if eq .Tab "fav"}}aria-current="page"{{end}}
        hx-get="{{.ListURL "fav"}}" hx-target="#list" hx-swap="innerHTML"
-       hx-push-url="{{.PageURL "fav"}}" hx-on::after-request="setActiveTab(this)">Favourites</a>
+       hx-push-url="{{.PageURL "fav"}}" hx-on::after-request="if (event.detail.successful) setActiveTab(this)">Favourites</a>
     <a href="{{.PageURL "archived"}}" class="{{if eq .Tab "archived"}}active{{end}}"
        {{if eq .Tab "archived"}}aria-current="page"{{end}}
        hx-get="{{.ListURL "archived"}}" hx-target="#list" hx-swap="innerHTML"
-       hx-push-url="{{.PageURL "archived"}}" hx-on::after-request="setActiveTab(this)">Archived</a>
+       hx-push-url="{{.PageURL "archived"}}" hx-on::after-request="if (event.detail.successful) setActiveTab(this)">Archived</a>
   </nav>
   </div>
 
+  {{/* Where a failure with no card to sit in gets reported: a tab switch, a
+       rotation, an expired session. filter.js fills it and unhides 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>
+
   {{template "setup" .}}
 
   {{template "keyrow" .}}
 
   {{template "recent" .}}
 
-  <main id="list" class="list">
+  <main id="list" class="list" tabindex="-1">
     {{template "list" .}}
   </main>
 </div>