Tighten fallback latency + alert UI on degraded states

Submit-first ordering. Patch 0004 now calls generator_submitblock
BEFORE writing the pending-block hex to disk. The happy path adds zero
disk I/O — we only dump when the primary returned false. The same
patch bounds generator_submitblock's "no live current_si" spin to
~3s instead of the original infinite loop, so a permanently-down
primary doesn't pin the stratifier; the bounded spin lets the caller
return false and lets local_block_submit dump for kamado-api to take
over.

Default grace lowered from 30s to 3s. With ckpool's bounded spin and
sub-second sweep cadence, the fallback now reacts within ~4s of a
failed primary submit — fast enough that the work is still relevant
for the current chain tip. The submitter's sweep poll dropped to 1s
to match.

UI HealthBanners. New top-of-page strip surfaces:
  * Fallback used (red banner, 24h after most recent event):
    "primary bitcoind didn't accept; backup X took over Y ago"
  * Submit gap (orange banner, only when no recent fallback):
    "N blocks attempted but unconfirmed — configure backups"
  * ZMQ stale (orange banner): no hashblock frame in 30+ minutes
Operators see degraded-but-not-fatal states without checking logs.

Startup readiness gate. main now waits up to 8s on agg.Ready() before
starting the HTTP server so the very first /api/snapshot doesn't show
all-zero state during the aggregator's first refresh. Capped so a
permanently-down bitcoind can't block startup; /healthz is honest
about the degraded state once we do start serving.

ui/src/App.svelte

@@ -3,6 +3,7 @@
   import { connect, snap } from "./stores/snapshot.svelte";
   import { selection } from "./stores/selection.svelte";
   import Header from "./lib/Header.svelte";
+  import HealthBanners from "./lib/HealthBanners.svelte";
   import PoolOverview from "./lib/PoolOverview.svelte";
   import HashrateChart from "./lib/HashrateChart.svelte";
   import MinersTable from "./lib/MinersTable.svelte";
@@ -19,6 +20,10 @@
 <main>
   <Header />
 
+  {#if snap.data}
+    <HealthBanners />
+  {/if}
+
   {#if !snap.data}
     <div class="card placeholder">
       <div class="stat-label">Status</div>