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/lanes.html

@@ -10,7 +10,7 @@
      this template only prints them. */}}
 {{define "lanes"}}
 <section class="lanes" id="lanes"
-         hx-get="/ui/admin/lanes" hx-trigger="every 30s" hx-swap="outerHTML">
+         hx-get="/ui/admin/lanes" hx-trigger="every 30s" hx-swap="outerHTML" hx-sync="this:replace">
   <div class="sechead">
     <h2 class="sec">Poll Lanes</h2>
     <p class="statusline">
@@ -18,20 +18,23 @@
       {{else}}Browser: {{if not .BrowserConfigured}}<span class="mark-faint">not configured</span>{{else if .BrowserReachable}}<span class="mark mark-strong">reachable</span>{{else}}<span class="mark bad">unreachable</span>{{end}}{{end}}
     </p>
   </div>
+  <p class="hint">Healthy idle (paused, browser asleep, nothing eligible) needs no action — attention needs the owner.</p>
   {{if .Rows}}
   <div class="tbl lanes">
     <div class="thead">
       <span>Site</span><span>Due</span><span>Checked</span><span>Gap</span>
-      <span>Last pass</span><span>Outcomes · state</span><span></span>
+      <span>Last pass</span><span title="Window outcomes for this Site (last 12h)">Outcomes · status</span><span></span>
     </div>
     {{range .Rows}}
     <div class="trow{{if .Attention}} attention{{end}}">
       <a class="c-site" href="{{.FailingHref}}">{{.Site}}</a>
-      <span>{{.Due}}</span>
-      <span>{{.Checked}}</span>
-      <span>{{.Gap}}</span>
+      {{/* data-label is the column word for the phone layout, which drops the
+           thead: three bare numbers in a row say nothing. */}}
+      <span data-label="due">{{.Due}}</span>
+      <span data-label="checked">{{.Checked}}</span>
+      <span data-label="gap">{{.Gap}}</span>
       <span>ran {{.Ran}}</span>
-      <span class="c-skip">{{if .HasChips}}{{range $i, $c := .Chips}}{{if $i}}<span class="mark-faint"> · </span>{{end}}<span class="mark">{{$c.Name}} {{$c.Count}}</span>{{end}}{{else}}<span class="mark-faint">none observed</span>{{end}}{{if .StatePhrase}} · <span class="{{if .StateGood}}ok{{else}}bad{{end}}">{{.StatePhrase}}</span>{{end}}</span>
+      <span class="c-skip">{{if .HasChips}}{{range $i, $c := .Chips}}{{if $i}}<span class="mark-faint"> · </span>{{end}}<span class="mark">{{$c.Name}} {{$c.Count}}</span>{{end}}{{else}}<span class="mark-faint">none observed</span>{{end}}{{if .StatePhrase}} · <span class="{{if .StateGood}}ok{{else}}bad{{end}}" aria-label="{{if .StateGood}}healthy: {{end}}{{.StatePhrase}}">{{.StatePhrase}}</span>{{end}}</span>
       {{/* The pause control lives in the one slot the design leaves for it:
            a running Lane offers the three durations and Pause; a paused Lane
            offers Resume in the same place. Pause is not destruction — it
@@ -39,11 +42,11 @@
            confirm row or the danger accent. The form wraps the select so the
            offered duration travels with the press. */}}
       <span class="c-ctrl">{{if .Paused}}<span class="pausebar">
-        <form hx-post="/admin/lanes/{{.Site}}/resume" hx-target="#lanes" hx-swap="outerHTML">
+        <form hx-post="/admin/lanes/{{.Site}}/resume" hx-target="#lanes" hx-swap="outerHTML" hx-indicator="#lanes" hx-disabled-elt="find button" hx-sync="closest #lanes:replace">
           <button type="submit" class="ghost">Resume</button>
         </form>
       </span>{{else}}<span class="pausebar">
-        <form hx-post="/admin/lanes/{{.Site}}/pause" hx-target="#lanes" hx-swap="outerHTML">
+        <form hx-post="/admin/lanes/{{.Site}}/pause" hx-target="#lanes" hx-swap="outerHTML" hx-indicator="#lanes" hx-disabled-elt="find button, select" hx-sync="closest #lanes:replace">
           <select name="duration" aria-label="Pause duration">
             <option>1h</option><option selected>6h</option><option>24h</option>
           </select>
@@ -57,4 +60,4 @@
   <p class="empty">No data yet — no Lane has recorded a pass.</p>
   {{end}}
 </section>
-{{end}}
+{{end}}