Fix Live Activity lock screen rendering and polish multi-account auth.

Flatten widget layouts and use system colors so banner and expanded regions render on vibrant lock screens; migrate auth sessions per account to prevent double sign-in; scope app lock PIN to accounts; default clock description to "Clock In"; add architecture docs and deferred form validation on auth screens.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/(app)/invoices/edit/[id].tsx

@@ -22,6 +22,7 @@ import { fonts, spacing } from "@/constants/theme";
 import { useAppTheme } from "@/contexts/ThemeContext";
 import { formatCurrency } from "@/lib/format";
 import { getInvoiceStatus } from "@/lib/invoice-status";
+import { validateLineItems } from "@/lib/form-validation";
 import { useTabBarScrollPadding } from "@/lib/tab-bar-insets";
 import type { ThemeColors } from "@/lib/theme-palette";
 import { useThemedStyles } from "@/lib/use-themed-styles";
@@ -100,6 +101,8 @@ export default function InvoiceEditScreen() {
   const taxAmount = subtotal * (taxRate / 100);
   const total = subtotal + taxAmount;
   const currency = invoice?.currency ?? "USD";
+  const lineItemsError = validateLineItems(items);
+  const canSave = !lineItemsError;
 
   if (!id) {
     return <LoadingScreen message="Invalid invoice" />;
@@ -170,6 +173,7 @@ export default function InvoiceEditScreen() {
   }
 
   function handleSave() {
+    if (!canSave) return;
     setError(null);
 
     const parsedItems: Array<{
@@ -180,25 +184,11 @@ export default function InvoiceEditScreen() {
     }> = [];
 
     for (const item of items) {
-      const hours = Number(item.hours);
-      const rate = Number(item.rate);
-      if (!item.description.trim()) {
-        setError("Each line needs a description");
-        return;
-      }
-      if (Number.isNaN(hours) || hours < 0) {
-        setError("Hours must be a valid number");
-        return;
-      }
-      if (Number.isNaN(rate) || rate < 0) {
-        setError("Rate must be a valid number");
-        return;
-      }
       parsedItems.push({
         date: item.date,
         description: item.description.trim(),
-        hours,
-        rate,
+        hours: Number(item.hours),
+        rate: Number(item.rate),
       });
     }
 
@@ -274,10 +264,16 @@ export default function InvoiceEditScreen() {
             </View>
           </Card>
 
+          {lineItemsError ? <Text style={styles.error}>{lineItemsError}</Text> : null}
           {error ? <Text style={styles.error}>{error}</Text> : null}
 
           <View style={styles.actions}>
-            <Button title="Save changes" loading={updateInvoice.isPending} onPress={handleSave} />
+            <Button
+              title="Save changes"
+              loading={updateInvoice.isPending}
+              disabled={!canSave}
+              onPress={handleSave}
+            />
             {status !== "paid" ? (
               <Button
                 title={status === "draft" ? "Send invoice" : "Resend invoice"}