feat: wire initialData/placeholderData into QoraBuilder and useQuery for instant SSR renders (#19)

[donfreddy]

Closes #19

Description

Eagerly seeds the cache with QoraOptions.initialData / placeholderData before the first render in QoraBuilder and useQuery. Eliminates the Initial loading flash, enabling instant renders with SSR/hydration data or placeholder content.

Changes

• QoraClient.prepopulateCache<T>(key, options) - new public method that eagerly creates a cache entry and applies initialData/placeholderData before any fetch.
• QoraBuilder - calls prepopulateCache in didChangeDependencies before _subscribe(), so the first build() sees the seeded Success state.
• useQuery hook - eagerly seeds the cache before useState initialization, so the first render shows pre-populated data.
• Tests - 3 dart tests for prepopulateCache + 2 Flutter tests for QoraBuilder with initialData
• Docs EN/FR updated with initialData seeding pattern in QoraBuilder documentation
• Changelogs updated for both qora and qora_flutter