Track cumulative work via accounted_diff_shares, surface round effort

Total work was summing pool.Shares (ckpool's accounted_shares — raw
per-share count) and multiplying by 2^32, which is nonsense: each
share's actual difficulty was ignored, so a pool running at any
real hashrate would show a tiny number. Switch to pool.Accepted,
which ckpool exposes as accounted_diff_shares (sum of each accepted
share's difficulty in diff-1-normalized units). cumulative_shares *
2^32 is now actually total hashes.

Bump the kv key from "cumulative_shares" to "cumulative_work" so
any value saved under the old name is orphaned rather than mixed
into the new (correctly-unit'd) counter on upgrade.

Total work tile's sub-info now shows the round effort:
cumulative_shares / network_difficulty * 100. Matches ckpool's own
formula at stratifier.c:8201 for the percent-of-block display.
Drop the duplicate effort text from the Expected block tile, which
was an uptime-based approximation of the same thing.

api/internal/state/aggregator.go

@@ -76,7 +76,13 @@ type Snapshot struct {
 
 const (
 	maxHistoryPoints = 1440 // 24h at 1 sample/min
-	kvCumulativeWork = "cumulative_shares"
+
+	// kvCumulativeWork is versioned: "cumulative_shares" (v1) accidentally
+	// summed pool.shares (raw per-share count), which is meaningless for
+	// hashrate math. "cumulative_work" (v2) sums pool.accepted — the
+	// diff-1-normalized work, so cumulative_work * 2^32 is real hashes.
+	// The old key is orphaned in the kv table on upgrade; harmless.
+	kvCumulativeWork = "cumulative_work"
 )
 
 // Aggregator refreshes a Snapshot on a ticker.
@@ -285,13 +291,16 @@ func (a *Aggregator) refresh(ctx context.Context) {
 	now := time.Now()
 
 	// --- cumulative work tracking ---
-	// Integrate only positive deltas on pool.Shares. A decrease means
-	// ckpool restarted (or its state reset); reset the baseline without
-	// touching the accumulated total. The first observation after load
-	// only establishes the baseline — those shares were already
-	// accumulated before the previous shutdown.
+	// Integrate only positive deltas on pool.Accepted, which is ckpool's
+	// accounted_diff_shares — the sum of each accepted share's
+	// difficulty, in diff-1-normalized units. pool.Shares is the raw
+	// per-share count and is useless for hashrate math. A decrease
+	// means ckpool restarted (or its state reset); we reset the
+	// baseline without touching the accumulated total. The first
+	// observation after load only establishes the baseline — the
+	// current counter was already integrated before we shut down.
 	if next.Pool != nil {
-		cur := float64(next.Pool.Shares)
+		cur := float64(next.Pool.Accepted)
 		if a.hasPoolSharesBaseline && cur >= a.lastPoolShares {
 			a.cumulativeShares += cur - a.lastPoolShares
 		}