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/discord.go

@@ -42,6 +42,11 @@ type DiscordConfig struct {
 	ClientID     string
 	ClientSecret string
 	GuildID      string
+	// GuildName is a human-readable name for the guild that gates access.
+	// It is never fetched from Discord; it is an optional display string the
+	// operator sets (DISCORD_GUILD_NAME) so the login screen can name the
+	// community without inventing one.
+	GuildName string
 	// RequiredRole, when non-empty, is a role ID a member must hold on top of
 	// guild membership. Empty by default: membership alone suffices.
 	RequiredRole string
@@ -173,16 +178,21 @@ func (h *Handler) discordCallback(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	// The refusal is the same for a non-member and a member without the
-	// required role, and it names neither the guild nor its id: an outsider
-	// cannot tell whether the guild exists, let alone which one gates.
+	// required role. When DISCORD_GUILD_NAME is set the message names the
+	// community so a stranger knows which Discord to ask about; otherwise it
+	// degrades to a generic community label and still gives the next step. It
+	// never names the numeric guild id, which would not be actionable.
 	//
 	// It also returns before EnsureReader, so a refused sign-in leaves no
 	// Reader row behind — the gate is the only thing standing between guild
 	// membership and a library.
 	if !isMember || (h.discord.RequiredRole != "" && !slices.Contains(member.Roles, h.discord.RequiredRole)) {
 		h.limiter.Fail(ip, time.Now())
-		h.renderLogin(w, http.StatusForbidden,
-			"This Discord account is not a member of this community.")
+		msg := "This Discord account is not a member of this community. Ask a member for an invite and try again."
+		if h.discord.GuildName != "" {
+			msg = fmt.Sprintf("This Discord account is not a member of the %s Discord. Ask a member for an invite and try again.", h.discord.GuildName)
+		}
+		h.renderLogin(w, http.StatusForbidden, msg)
 		return
 	}