cospend: filter recent activity by chart category selection

Clicking a category on the bar chart now filters the recent activity
list to show only payments in that category. Includes a clear filter
button and empty state message. Also increases recent splits from 10
to 30 for better coverage when filtering.

src/lib/components/BarChart.svelte

@@ -2,7 +2,7 @@
   import { onMount } from 'svelte';
   import { Chart, registerables } from 'chart.js';
 
-  let { data = { labels: [], datasets: [] }, title = '', height = '400px' } = $props<{ data?: any, title?: string, height?: string }>();
+  let { data = { labels: [], datasets: [] }, title = '', height = '400px', onFilterChange = null } = $props<{ data?: any, title?: string, height?: string, onFilterChange?: ((categories: string[] | null) => void) | null }>();
 
   let canvas = $state();
   let chart = $state();
@@ -42,6 +42,19 @@
     return categoryColorMap[category] || nordColors[index % nordColors.length];
   }
 
+  function emitFilter() {
+    if (!onFilterChange || !chart) return;
+    const allVisible = chart.data.datasets.every((_, idx) => !chart.getDatasetMeta(idx).hidden);
+    if (allVisible) {
+      onFilterChange(null);
+    } else {
+      const visible = chart.data.datasets
+        .filter((_, idx) => !chart.getDatasetMeta(idx).hidden)
+        .map(ds => ds.label.toLowerCase());
+      onFilterChange(visible);
+    }
+  }
+
   function createChart() {
     if (!canvas || !data.datasets) return;
 
@@ -135,7 +148,6 @@
             },
             onClick: (event, legendItem, legend) => {
               const datasetIndex = legendItem.datasetIndex;
-              const clickedMeta = chart.getDatasetMeta(datasetIndex);
 
               // Check if only this dataset is currently visible
               const onlyThisVisible = chart.data.datasets.every((dataset, idx) => {
@@ -156,6 +168,7 @@
               }
 
               chart.update();
+              emitFilter();
             }
           },
           title: {
@@ -229,6 +242,7 @@
             }
 
             chart.update();
+            emitFilter();
           }
         }
       },

src/routes/api/cospend/balance/+server.ts

@@ -89,7 +89,7 @@ export const GET: RequestHandler = async ({ locals, url }) => {
         },
         { $unwind: '$paymentId' },
         { $sort: { 'paymentId.date': -1, 'paymentId.createdAt': -1 } },
-        { $limit: 10 }
+        { $limit: 30 }
       ]);
 
       // For settlements, fetch the other user's split info

src/routes/cospend/+page.svelte

@@ -24,6 +24,13 @@
   let error = $state(null);
   let monthlyExpensesData = $state(data.monthlyExpensesData || { labels: [], datasets: [] });
   let expensesLoading = $state(false);
+  let categoryFilter = $state(null);
+
+  let filteredSplits = $derived(
+    categoryFilter
+      ? (balance.recentSplits || []).filter(split => categoryFilter.includes(split.paymentId?.category))
+      : balance.recentSplits || []
+  );
 
   // Component references for refreshing
   let enhancedBalanceComponent;
@@ -177,6 +184,7 @@
           data={monthlyExpensesData}
           title="Monthly Expenses by Category"
           height="400px"
+          onFilterChange={(categories) => categoryFilter = categories}
         />
       {:else}
         <div class="loading">
@@ -194,9 +202,17 @@
     <div class="error">Error: {error}</div>
   {:else if balance.recentSplits && balance.recentSplits.length > 0}
     <div class="recent-activity">
-      <h2>Recent Activity</h2>
+      <div class="recent-activity-header">
+        <h2>Recent Activity{#if categoryFilter} <span class="filter-label">— {categoryFilter.map(c => getCategoryName(c)).join(', ')}</span>{/if}</h2>
+        {#if categoryFilter}
+          <button class="clear-filter" onclick={() => categoryFilter = null}>Clear filter</button>
+        {/if}
+      </div>
+      {#if filteredSplits.length === 0}
+        <p class="no-results">No recent activity in {categoryFilter.map(c => getCategoryName(c)).join(', ')}.</p>
+      {/if}
       <div class="activity-dialog">
-        {#each balance.recentSplits as split}
+        {#each filteredSplits as split}
           {#if isSettlementPayment(split.paymentId)}
             <!-- Settlement Payment Display - User -> User Flow -->
             <a
@@ -363,11 +379,49 @@
     margin-right: auto;
   }
 
-  .recent-activity h2 {
+  .recent-activity-header {
+    display: flex;
+    justify-content: space-between;
+    align-items: baseline;
+    gap: 1rem;
     margin-bottom: 1rem;
+  }
+
+  .recent-activity h2 {
+    margin: 0;
     color: var(--nord0);
   }
 
+  .filter-label {
+    font-weight: 400;
+    font-size: 1rem;
+    color: var(--nord3);
+  }
+
+  .clear-filter {
+    background: none;
+    border: 1px solid var(--nord4);
+    border-radius: 0.5rem;
+    padding: 0.25rem 0.75rem;
+    color: var(--nord3);
+    cursor: pointer;
+    font-size: 0.85rem;
+    white-space: nowrap;
+    transition: all 0.2s;
+  }
+
+  .clear-filter:hover {
+    border-color: var(--blue);
+    color: var(--blue);
+  }
+
+  .no-results {
+    text-align: center;
+    color: var(--nord3);
+    font-style: italic;
+    padding: 1rem 0;
+  }
+
   @media (prefers-color-scheme: dark) {
     .recent-activity {
       background: var(--accent-dark);
@@ -377,6 +431,24 @@
     .recent-activity h2 {
       color: var(--font-default-dark);
     }
+
+    .filter-label {
+      color: var(--nord4);
+    }
+
+    .clear-filter {
+      border-color: var(--nord3);
+      color: var(--nord4);
+    }
+
+    .clear-filter:hover {
+      border-color: var(--blue);
+      color: var(--blue);
+    }
+
+    .no-results {
+      color: var(--nord4);
+    }
   }
 
   .activity-dialog {