hide the rescan control on a source that cannot rescan

the card keyed off `sync.rescan` being null, which is what esplora
reports — and equally what nbxplorer reports before its first scan. so
the guard could not tell "unsupported" from "not yet run" and resolved
it the wrong way: every esplora wallet got a button that 501s.

canRescan carries the distinction explicitly.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/servers/sidecar/wallet/backends/onchain.ts

@@ -313,6 +313,7 @@ export class OnchainBackend extends BaseBackend {
       syncedAt: this.current?.at ?? null,
       stale: this.current !== null && Date.now() - this.current.at >= SCAN_TTL_MS,
       lastError: this.lastError,
+      canRescan: this.chain.startRescan !== undefined,
       rescan: this.chain.rescanState?.() ?? null,
     };
   }

src/servers/sidecar/wallet/types.ts

@@ -321,6 +321,14 @@ export type SyncState = {
   stale: boolean;
   /** The last refresh failure, still reported while the previous good data is being served. */
   lastError: string | null;
+  /**
+   * Whether this chain source can search the chain from scratch at all. False for Esplora, which asks
+   * about every address on every refresh and so has nothing to catch up on.
+   *
+   * Distinct from `rescan === null`, which is ALSO what a source that simply hasn't run one yet reports
+   * — conflating the two put a button that 501s in front of every Esplora wallet.
+   */
+  canRescan: boolean;
   /** A deep rescan in flight, or the outcome of the last one. Null from a source that cannot rescan. */
   rescan: RescanState | null;
 };