Fix linked account balance currency mismatch (#566)

* Fix linked account balance currency mismatch

When linking accounts from providers (Lunchflow, SimpleFIN, Enable Banking),
the initial sync was creating balances before the correct currency was known.
This caused:
1. Opening anchor entry created with default currency (USD/EUR)
2. First sync created balances with wrong currency
3. Later syncs created balances with correct currency
4. Both currency balances existed, charts showed wrong (zero) values

Changes:
- Add `skip_initial_sync` parameter to `Account.create_and_sync`
- Skip initial sync for linked accounts (provider sync handles it)
- Add currency filter to ChartSeriesBuilder query to only fetch
  balances matching the account's current currency

* Add migration script and add tests

* Update schema.rb

---------

Signed-off-by: soky srm <sokysrm@gmail.com>
Co-authored-by: sokie <sokysrm@gmail.com>

app/controllers/enable_banking_items_controller.rb

@@ -269,13 +269,17 @@ class EnableBankingItemsController < ApplicationController
           end
 
           # Create the internal Account (uses save! internally, will raise on failure)
+          # Skip initial sync - provider sync will handle balance creation with correct currency
           account = Account.create_and_sync(
-            family: Current.family,
-            name: enable_banking_account.name,
-            balance: enable_banking_account.current_balance || 0,
-            currency: enable_banking_account.currency || "EUR",
-            accountable_type: accountable_type,
-            accountable_attributes: {}
+            {
+              family: Current.family,
+              name: enable_banking_account.name,
+              balance: enable_banking_account.current_balance || 0,
+              currency: enable_banking_account.currency || "EUR",
+              accountable_type: accountable_type,
+              accountable_attributes: {}
+            },
+            skip_initial_sync: true
           )
 
           # Link account to enable_banking_account via account_providers