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.

README.md

@@ -48,6 +48,7 @@ covers are stored, so the library renders in full with the browser switched off.
 | `DISCORD_CLIENT_ID` | *(required)* | Discord application credentials for the browser sign-in (ADR-0002). |
 | `DISCORD_CLIENT_SECRET` | *(required)* | As above. Never logged, never echoed in an error. |
 | `DISCORD_GUILD_ID` | *(required)* | The one guild whose membership gates sign-in, checked at login only. Membership *is* registration: any member becomes a Reader on first login. |
+| `DISCORD_GUILD_NAME` | empty | Human-readable name for `DISCORD_GUILD_ID`, shown on the login screen so a stranger knows which community owns this library and who to ask for an invite. When unset the page shows a generic "private community" label. |
 | `DISCORD_REDIRECT_URI` | *(required)* | Exact callback URL; Discord matches it verbatim against the registered redirect. |
 | `DISCORD_REQUIRED_ROLE` | empty | Role snowflake a member must additionally hold. Empty means guild membership alone suffices. |
 | `DISCORD_API_BASE` | `https://discord.com/api/v10` | Test seam — tests point it at a local stub so the real token exchange runs. |