Record and display winning share difficulty on found blocks

Logmon now captures the share diff from ckpool's "Possible block
solve" line preceding the confirmation and attaches it to the
BlockEvent. Persisted as share_diff alongside height/hash/reward
and rendered as a new column in the dashboard block history.

ui/src/types.ts

@@ -85,6 +85,7 @@ export type BlockRecord = {
   reward_btc?: number;
   found_at: string; // RFC 3339
   source: string;
+  share_diff?: number;
 };
 
 export type Snapshot = {