refactor(frontend): migrate Activities.tsx to the banner system

Every load/classify/unlock failure now calls showError instead of
setting local error state; the inline error paragraph is removed.

frontend/src/pages/Activities.tsx

@@ -1,5 +1,6 @@
 import { useCallback, useEffect, useRef, useState } from "react";
 import { api } from "../api/client";
+import { showError } from "../banner";
 import { ExpectedVsActualChart } from "../components/charts/ExpectedVsActualChart";
 import { formatMinutesSeconds } from "../components/PaceField";
 import { RawDataModal } from "../components/RawDataModal";
@@ -164,7 +165,6 @@ export function Activities() {
   const [kinds, setKinds] = useState<WorkoutKind[]>([]);
   const [profile, setProfile] = useState<Profile | null>(null);
   const [filterKindId, setFilterKindId] = useState<string>("");
-  const [error, setError] = useState<string | null>(null);
   const [resolvingId, setResolvingId] = useState<number | null>(null);
   const [rawDataItem, setRawDataItem] = useState<ReviewQueueItem | null>(null);
 
@@ -211,7 +211,7 @@ export function Activities() {
         setItems((prev) => [...prev, ...page.items]);
         setNextCursor(page.next_cursor);
       } catch (e) {
-        if (gen === generationRef.current) setError(String(e));
+        if (gen === generationRef.current) showError(String(e));
       } finally {
         setLoadingMore(false);
         inFlightRef.current = null;
@@ -240,7 +240,7 @@ export function Activities() {
         if (filter === "") setGrandTotal(page.total);
       })
       .catch((e) => {
-        if (gen === generationRef.current) setError(String(e));
+        if (gen === generationRef.current) showError(String(e));
       })
       .finally(() => {
         if (gen === generationRef.current) setInitialLoading(false);
@@ -249,8 +249,8 @@ export function Activities() {
 
   useEffect(() => {
     loadFirstPage("");
-    api.listWorkoutKinds().then(setKinds).catch((e) => setError(String(e)));
-    api.getProfile().then(setProfile).catch((e) => setError(String(e)));
+    api.listWorkoutKinds().then(setKinds).catch((e) => showError(String(e)));
+    api.getProfile().then(setProfile).catch((e) => showError(String(e)));
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, []);
 
@@ -289,7 +289,7 @@ export function Activities() {
           .filter((i) => matchesFilter(i, filterKindId)),
       );
     } catch (e) {
-      setError(String(e));
+      showError(String(e));
     } finally {
       setResolvingId(null);
     }
@@ -311,7 +311,7 @@ export function Activities() {
           .filter((i) => matchesFilter(i, filterKindId)),
       );
     } catch (e) {
-      setError(String(e));
+      showError(String(e));
     } finally {
       setResolvingId(null);
     }
@@ -327,7 +327,7 @@ export function Activities() {
           .filter((i) => matchesFilter(i, filterKindId)),
       );
     } catch (e) {
-      setError(String(e));
+      showError(String(e));
     } finally {
       setResolvingId(null);
     }
@@ -341,8 +341,6 @@ export function Activities() {
 
   return (
     <div className="page">
-      {error && <p className="error">{error}</p>}
-
       {grandTotal > 0 && (
         <div className="filter-pills">
           <button