Add balance snapshots and reconciliation calculations

[theMattCode]

User story

As a user, I want to enter known account balances and see reconciliation differences so I can start using the system without backfilling all historical activity.

Depends on

• Issue Add financial account APIs and account management UI #13

Scope

• Add backend APIs for creating and listing balance snapshots per financial account.
• Calculate current balance as latest balance snapshot plus later ledger entries.
• Calculate reconciliation differences where a later snapshot does not match the calculated balance.
• Add account detail UI for snapshots, calculated balance, latest confirmed balance, and reconciliation difference.
• Add tests for asset and liability balance signs.

Non-scope

• Automatic adjustment entries.
• Ledger entry UI.
• Net worth dashboard composition.

Acceptance criteria

• User can enter initial and later balance snapshots.
• Current balance uses snapshot-plus-ledger calculation.
• Reconciliation differences are visible and not hidden by generated adjustments.
• Credit and loan balances are treated as liabilities in balance/net worth calculations.