Unify best-share semantics across leaderboard / miners / user page

BestShares read worker.bestdiff for its "Best (round)" column,
while MinersTable used client.bestdiff for online workers (and
worker.bestdiff only as a fallback for offline ones). ckpool
resets *both* fields in reset_bestshares() when the pool finds a
block, but reconnect / timing can leave them briefly divergent,
so the same worker could show different values in the two tables.

Switch every "session best" display to the same computation:

    max(client.bestdiff, worker.bestdiff)

…picking whichever is currently higher. Relabel the column from
"Best (round)" to "Best (session)" in MinersTable, BestShares, and
UserDetailPage so the meaning matches the value. UserDetailPage's
per-user summary tile used to show the user's round-best as a
sub-line; drop it, since it contradicts the "session" framing at
the row level.

Net effect: scroll between the leaderboard and the miners table
and the same worker's best-share number stays put.

ui/src/lib/UserDetailPage.svelte

@@ -49,7 +49,12 @@
         hashrate1m: (c?.dsps1 ?? w.dsps1) * 2 ** 32,
         hashrate1h: (c?.dsps60 ?? w.dsps60) * 2 ** 32,
         diff: c?.diff ?? w.mindiff,
-        bestRound: c?.bestdiff ?? w.bestdiff,
+        // Session best = larger of client.bestdiff (this stratum
+        // connection) and worker.bestdiff (this worker in the
+        // current round). Matches the MinersTable / BestShares
+        // leaderboard semantics so values are consistent across
+        // screens.
+        bestRound: Math.max(c?.bestdiff ?? 0, w.bestdiff),
         bestEver: w.bestever,
         lastShare: c?.lastshare ?? w.lastshare,
         online: !!c,
@@ -138,7 +143,7 @@
     <div class="card">
       <div class="stat-label">Best share (ever)</div>
       <div class="stat-value">{formatDifficulty(totals.bestEver || totals.bestDiff)}</div>
-      <div class="stat-sub">round best {formatDifficulty(totals.bestDiff)}</div>
+      <div class="stat-sub">across all this user's workers</div>
     </div>
     <div class="card">
       <div class="stat-label">Workers</div>
@@ -166,7 +171,7 @@
               <th class="num">Hashrate (1m)</th>
               <th class="num">Hashrate (1h)</th>
               <th class="num">Diff</th>
-              <th class="num">Best (round)</th>
+              <th class="num">Best (session)</th>
               <th class="num">Best (ever)</th>
               <th class="num">Last share</th>
             </tr>