## What
When the add/edit transaction dialog resets in **create** mode, it no
longer auto-selects the first transactional account. The account field
starts empty so the user must pick one deliberately.
The only exception is the account detail page (`accounts/{uuid}`), which
passes `initialAccountId`; there the dialog still pre-selects that
account.
## Why
Auto-selecting the first account made it easy to accidentally book a
transaction to the wrong account. Forcing a deliberate pick (except
where the account is unambiguous from context) avoids that.
## How
- `edit-transaction-dialog.tsx`: dropped the `availableAccounts[0]`
fallback. The field now resolves to `initialAccount?.id ?? ''`.
- The existing submit guard (`Account is required`) already covers the
empty case.
- `Accounts/Show.tsx` already passes `initialAccountId={account.id}`, so
the account-page behavior is unchanged; the transactions list passes
nothing, so it now starts empty.
## Tests
Added two unit tests to `edit-transaction-dialog.test.tsx`:
- no `initialAccountId` → account value stays empty (no auto-select)
- matching `initialAccountId` → account is pre-selected
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