Revamp dashboard and persist pool stats across restarts

Dashboard now renders 10 tiles in a 5x2 overview: hashrate, best
share, miners, network hashrate, and expected block on the top row;
difficulty, block height, block reward, total work, and the
difficulty-adjustment countdown on the bottom row. Difficulty is
rendered with T/P suffixes instead of scientific notation, the main
hashrate card shows the 1-minute value, and the block-height tile
pulses orange when the network tip advances.

Added a 24-hour hashrate area chart below the overview, sampled
once per minute. Samples are persisted to a new hashrate_samples
SQLite table and restored on startup so the chart doesn't reset
every time kamado-api is restarted.

Cumulative pool work (sum of accepted diff-1-normalized shares) is
now tracked across ckpool restarts. The aggregator integrates only
positive deltas on pool.Shares — a regression means ckpool's
counter reset to zero and the baseline is refreshed without losing
the running total. A hasPoolSharesBaseline flag prevents double-
counting on the first refresh after a kamado-api restart. The
value is persisted to a new kv table once per minute.

Next-block reward (subsidy + fees) is fetched from bitcoind
getblocktemplate at most once per minute and surfaced as a tile.

Header's block-height badge now reads prevHeight via untrack() so
the effect doesn't form a dependency cycle with its own write.

ui/src/App.svelte

@@ -3,6 +3,7 @@
   import { connect, snap } from "./stores/snapshot.svelte";
   import Header from "./lib/Header.svelte";
   import PoolOverview from "./lib/PoolOverview.svelte";
+  import HashrateChart from "./lib/HashrateChart.svelte";
   import MinersTable from "./lib/MinersTable.svelte";
   import BlocksTable from "./lib/BlocksTable.svelte";
   import BestShares from "./lib/BestShares.svelte";
@@ -25,6 +26,7 @@
     </div>
   {:else}
     <PoolOverview />
+    <HashrateChart />
     <div class="grid grid-2">
       <BlocksTable />
       <BestShares />