Quality-of-life enhancements: CSV import/export, opening balances, interest rates, mobile UX

- CSV import/export for Units, Projects, and Vendors with match-on-name/number upsert
- Cash Flow report toggle for Cash Only vs Cash + Investments
- Per-account and bulk opening balance setting with as-of date
- Interest rate field on normal accounts with estimated monthly/annual interest display
- Mobile sidebar auto-close on navigation
- Shared CSV parsing/export utility extracted to frontend/src/utils/csv.ts

DB migration needed for existing tenants:
  ALTER TABLE accounts ADD COLUMN IF NOT EXISTS interest_rate DECIMAL(6,4);

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

frontend/src/utils/csv.ts

@@ -0,0 +1,84 @@
+/**
+ * Shared CSV parsing and export utilities.
+ */
+
+/** Parse CSV text into an array of objects keyed by lowercase header names. */
+export function parseCSV(text: string): Record<string, string>[] {
+  const lines = text.trim().split('\n');
+  if (lines.length < 2) return [];
+
+  // Strip leading * from headers (used to mark required fields)
+  const headers = lines[0].split(',').map((h) => h.trim().replace(/^\*/, '').toLowerCase());
+  const rows: Record<string, string>[] = [];
+
+  for (let i = 1; i < lines.length; i++) {
+    const line = lines[i].trim();
+    if (!line) continue;
+
+    // Handle quoted fields containing commas
+    const values: string[] = [];
+    let current = '';
+    let inQuotes = false;
+    for (let j = 0; j < line.length; j++) {
+      const ch = line[j];
+      if (ch === '"') {
+        inQuotes = !inQuotes;
+      } else if (ch === ',' && !inQuotes) {
+        values.push(current.trim());
+        current = '';
+      } else {
+        current += ch;
+      }
+    }
+    values.push(current.trim());
+
+    const row: Record<string, string> = {};
+    headers.forEach((h, idx) => {
+      row[h] = values[idx] || '';
+    });
+    rows.push(row);
+  }
+
+  return rows;
+}
+
+/** Convert an array of objects to CSV text and trigger a browser download. */
+export function downloadCSV(rows: Record<string, any>[], headers: string[], filename: string) {
+  const csvLines = [headers.join(',')];
+  for (const row of rows) {
+    const values = headers.map((h) => {
+      const key = h.replace(/^\*/, '').toLowerCase();
+      const val = row[key] ?? '';
+      const str = String(val);
+      // Quote values that contain commas or quotes
+      if (str.includes(',') || str.includes('"') || str.includes('\n')) {
+        return `"${str.replace(/"/g, '""')}"`;
+      }
+      return str;
+    });
+    csvLines.push(values.join(','));
+  }
+
+  const blob = new Blob([csvLines.join('\n')], { type: 'text/csv' });
+  const url = window.URL.createObjectURL(blob);
+  const a = document.createElement('a');
+  a.href = url;
+  a.download = filename;
+  document.body.appendChild(a);
+  a.click();
+  document.body.removeChild(a);
+  window.URL.revokeObjectURL(url);
+}
+
+/** Download a blob response from the API as a file. */
+export function downloadBlob(data: Blob, filename: string, type = 'text/csv') {
+  const blob = new Blob([data], { type });
+  const url = window.URL.createObjectURL(blob);
+  const a = document.createElement('a');
+  a.href = url;
+  a.download = filename;
+  document.body.appendChild(a);
+  a.click();
+  document.body.removeChild(a);
+  window.URL.revokeObjectURL(url);
+}