Replace per-kind reclassify with a single global reclassify that respects manual and Race locks

Manual assignments are the user's definitive word and Race assignments come
from a hard Garmin fact (eventType.typeKey), not a retunable rule -- neither
is ever touched by reclassify again, and Race can no longer be set by hand
via the review queue. Adds an Activities page so this locked/unlocked status
is visible per workout, since Review Queue only ever showed unresolved items.

frontend/src/pages/Activities.tsx

@@ -0,0 +1,73 @@
+import { useEffect, useState } from "react";
+import { api } from "../api/client";
+import type { ActivityListItem } from "../types/api";
+
+function formatPace(avgSpeedMps: number | null): string | null {
+  if (avgSpeedMps == null || avgSpeedMps <= 0) return null;
+  const secPerKm = 1000 / avgSpeedMps;
+  const m = Math.floor(secPerKm / 60);
+  const s = Math.round(secPerKm % 60);
+  return `${m}:${s.toString().padStart(2, "0")}/km`;
+}
+
+function lockReason(item: ActivityListItem): string {
+  if (item.assignment_source === "manual") return "Manually assigned -- kept as-is by reclassify";
+  if (item.workout_kind_name === "Race") return "Race, from Garmin metadata -- kept as-is by reclassify";
+  return "";
+}
+
+export function Activities() {
+  const [items, setItems] = useState<ActivityListItem[]>([]);
+  const [error, setError] = useState<string | null>(null);
+
+  useEffect(() => {
+    api.listActivities().then(setItems).catch((e) => setError(String(e)));
+  }, []);
+
+  return (
+    <div className="page">
+      <h2>Activities</h2>
+      {error && <p className="error">{error}</p>}
+
+      {items.length === 0 ? (
+        <p className="empty-state">No activities synced yet.</p>
+      ) : (
+        <table className="kinds-table">
+          <thead>
+            <tr>
+              <th>Date</th>
+              <th>Name</th>
+              <th>Distance</th>
+              <th>Pace</th>
+              <th>Kind</th>
+              <th>Source</th>
+              <th></th>
+            </tr>
+          </thead>
+          <tbody>
+            {items.map((item) => {
+              const pace = formatPace(item.AvgSpeedMps);
+              return (
+                <tr key={item.ID}>
+                  <td>{item.StartTimeUTC}</td>
+                  <td>{item.ActivityName || item.ActivityType}</td>
+                  <td>{(item.DistanceMeters / 1000).toFixed(2)} km</td>
+                  <td>{pace ?? "—"}</td>
+                  <td>{item.workout_kind_name ?? "—"}</td>
+                  <td>{item.assignment_source ?? "—"}</td>
+                  <td>
+                    {item.locked && (
+                      <span className="lock-badge" title={lockReason(item)}>
+                        Locked
+                      </span>
+                    )}
+                  </td>
+                </tr>
+              );
+            })}
+          </tbody>
+        </table>
+      )}
+    </div>
+  );
+}