Add 7-day sync overlap buffer and improve empty investments state

SimpleFIN sync now looks back 7 days from last sync to catch
retroactively posted transactions. Investments page shows account
balance instead of $0 summary cards when no holdings exist.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/views/investments/InvestmentsView.vue

@@ -29,6 +29,21 @@
       <v-progress-linear v-if="loading" indeterminate color="primary" class="mb-4" />
 
       <template v-if="performance && !loading">
+        <!-- No holdings: show account balance + prompt -->
+        <template v-if="performance.holdings.length === 0">
+          <v-card class="text-center pa-8">
+            <div class="text-h4 mb-2" :class="selectedAccountBalance >= 0 ? 'text-success' : 'text-error'">
+              {{ formatCurrency(selectedAccountBalance) }}
+            </div>
+            <div class="text-caption text-medium-emphasis mb-6">Account Balance</div>
+            <v-icon size="48" color="grey-lighten-1" class="mb-4">mdi-chart-pie</v-icon>
+            <p class="text-medium-emphasis">No individual holdings tracked for this account.</p>
+            <p class="text-medium-emphasis text-caption">Add holdings to see allocation, cost basis, and performance.</p>
+          </v-card>
+        </template>
+
+        <!-- Has holdings: show full portfolio view -->
+        <template v-else>
           <!-- Portfolio Summary -->
           <v-row class="mb-4">
             <v-col cols="12" sm="6" md="3">
@@ -67,7 +82,7 @@
 
           <!-- Allocation Chart + Holdings Table -->
           <v-row>
-          <v-col v-if="performance.holdings.length > 0" cols="12" md="4">
+            <v-col cols="12" md="4">
               <v-card>
                 <v-card-title>Allocation</v-card-title>
                 <v-card-text>
@@ -80,10 +95,10 @@
                 </v-card-text>
               </v-card>
             </v-col>
-          <v-col cols="12" :md="performance.holdings.length > 0 ? 8 : 12">
+            <v-col cols="12" md="8">
               <v-card>
                 <v-card-title>Holdings</v-card-title>
-              <v-card-text v-if="performance.holdings.length > 0">
+                <v-card-text>
                   <v-data-table
                     :headers="holdingsHeaders"
                     :items="performance.holdings"
@@ -107,13 +122,11 @@
                     </template>
                   </v-data-table>
                 </v-card-text>
-              <v-card-text v-else class="text-center pa-8 text-medium-emphasis">
-                No holdings found for this account. Add investment holdings to track your portfolio.
-              </v-card-text>
               </v-card>
             </v-col>
           </v-row>
         </template>
+      </template>
 
       <v-card v-if="!loading && !performance && selectedAccountId" class="text-center pa-8">
         <v-icon size="48" color="grey-lighten-1" class="mb-4">mdi-chart-line</v-icon>
@@ -158,6 +171,11 @@ const investmentAccounts = computed(() =>
     }))
 )
 
+const selectedAccountBalance = computed(() => {
+  const acct = investmentAccounts.value.find(a => a.id === selectedAccountId.value)
+  return acct?.balance ?? 0
+})
+
 // --- Allocation Chart ---
 const allocationChartSeries = computed(() =>
   performance.value?.holdings.map(h => h.marketValue) ?? []

src/Purrse.Api/Services/BankSyncService.cs

@@ -424,12 +424,15 @@ public class BankSyncService : IBankSyncService
 
         // For SimpleFIN, use the earliest start date across all accounts so the
         // single API call (cached) returns transactions covering every account.
+        // Subtract a 7-day overlap buffer because banks may post transactions with
+        // dates earlier than when they appear in the SimpleFIN feed. Duplicate
+        // detection (FitId + fingerprint) ensures re-fetched transactions are skipped.
         var endDate = DateTime.UtcNow;
         DateTime? simpleFinStartDate = null;
         if (connection.Provider == SyncProvider.SimpleFIN)
         {
             simpleFinStartDate = enabledAccounts
-                .Select(la => la.LastSyncAt ?? endDate.AddDays(-90))
+                .Select(la => la.LastSyncAt?.AddDays(-7) ?? endDate.AddDays(-90))
                 .Min();
         }
 
@@ -447,7 +450,7 @@ public class BankSyncService : IBankSyncService
 
             try
             {
-                var startDate = simpleFinStartDate ?? linkedAccount.LastSyncAt ?? endDate.AddDays(-90);
+                var startDate = simpleFinStartDate ?? linkedAccount.LastSyncAt?.AddDays(-7) ?? endDate.AddDays(-90);
 
                 var provider = connection.Provider == SyncProvider.Plaid
                     ? (Purrse.Plugins.Abstractions.IAccountSyncProvider)_plaid