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/MinersTable.svelte

@@ -35,14 +35,20 @@
       const wname = c.workername || `${c.address}.unnamed`;
       seen.add(wname);
       const w = byWorker.get(wname);
+      // "Session best" is the larger of the live stratum client's
+      // best_diff and the worker_instance's best_diff. ckpool's
+      // reset_bestshares() clears both when the pool finds a block,
+      // but reconnect / timing can leave them briefly divergent —
+      // max() keeps this column aligned with the leaderboard.
+      const sessionBest = Math.max(c.bestdiff, w?.bestdiff ?? 0);
       out.push({
         workerName: wname,
         user: c.address,
         hardware: detectHardware(c.useragent),
         hashrate1m: c.dsps1 * 2 ** 32,
         hashrate1h: c.dsps60 * 2 ** 32,
-        bestDiff: c.bestdiff,
-        bestEver: w?.bestever ?? 0,
+        bestDiff: sessionBest,
+        bestEver: w?.bestever ?? sessionBest,
         lastShare: c.lastshare,
         difficulty: c.diff,
         address: c.address,
@@ -88,7 +94,7 @@
             <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>