feat(savings,shares): bidirectional savings/share account linking, new Migrate Cash Denomination screen

- Savings account creation shows a required "Default Share Account" picker once Share Linking is
  enabled, populated from the client's active share accounts; submits shareAccountId alongside it.
- Fixes a real data-loss bug on Share Account creation: the Charges step re-seeded its table from
  the product's default charges on every ngOnChanges firing (e.g. stepping back to Details and
  forward again), silently discarding a charge the clerk had already added, edited, or removed - a
  pristine guard existed for exactly this but wasn't checked in ngOnChanges. Now it is.
- Removes the hard "required" on Share Account creation's Default Savings Account field, matching
  the backend now accepting a share account with no linked savings account.
- New /accounting/migrate-cash-denomination screen: counts a branch's opening cash into notes and
  coins, checks it against (or defines, if unset) the Cash GL account's balance, and opens the
  branch's first business day with that same count - the vault-side counterpart to Migrate Opening
  Balances, kept under Accounting rather than Branch Ops/Cashier so it isn't reachable by mistake.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/savings/create-savings-account/create-savings-account.component.ts

@@ -146,6 +146,13 @@ export class CreateSavingsAccountComponent {
       });
       return;
     }
+    if (detailsStep.shareLinkingEnabled && !detailsStep.shareAccountId) {
+      this.alertService.alert({
+        type: 'Error',
+        message: 'Share Linking is enabled — please select the share account to link before submitting.'
+      });
+      return;
+    }
     const locale = this.settingsService.language.code;
     const dateFormat = this.settingsService.dateFormat;
     const monthDayFormat = 'dd MMMM';
@@ -208,6 +215,14 @@ export class CreateSavingsAccountComponent {
             type: 'Error',
             message: 'An account with this External ID already exists. Please use a unique External ID.'
           });
+        } else if (
+          errorCode === 'error.msg.savingsaccount.share.account.required' ||
+          errorCode === 'error.msg.savingsaccount.share.account.not.active.for.client'
+        ) {
+          this.alertService.alert({
+            type: 'Error',
+            message: 'Please select an active share account belonging to this client before submitting.'
+          });
         }
       }
     });