fix: budget save error and add read-only view mode (v2026.03.10)

Fix budget save 500 error caused by three data mismatches between
frontend and backend: wrapped payload ({lines:[...]}) vs expected
raw array, snake_case vs camelCase field names (account_id vs
accountId), and dec_amt vs dec for December values.

Add read-only budget view as default for existing budgets with an
"Edit Budget" button to enter edit mode, and Cancel to discard
changes - reducing accidental edits.

Bump version to 2026.03.10 across all packages and settings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/pages/settings/SettingsPage.tsx

@@ -117,7 +117,7 @@ export function SettingsPage() {
             </Group>
             <Group justify="space-between">
               <Text size="sm" c="dimmed">Version</Text>
-              <Badge variant="light">2026.3.7 (Beta)</Badge>
+              <Badge variant="light">2026.03.10</Badge>
             </Group>
             <Group justify="space-between">
               <Text size="sm" c="dimmed">API</Text>