fix(sdk): persist theme preference and inject into all container globalProps

[Huxpro]

Closes #56

Problem

Sub-pages opened via navigate()/open() don't inherit the user's in-app theme preference. Each container is an isolated LynxView with its own JS runtime — theme is determined at creation from force_theme_style (if set) or the system dark mode setting. There's no mechanism to propagate an in-app preference across containers.

Solution

Add setPreferredTheme / getPreferredTheme APIs to the SDK, backed by native persistence:

iOS (SPKGlobalPropsUtils):

SPKGlobalPropsUtils.setPreferredTheme("dark")   // persist
SPKGlobalPropsUtils.preferredTheme()              // read

Android (GlobalPropsUtils):

GlobalPropsUtils.setPreferredTheme(context, "dark")  // persist
GlobalPropsUtils.getPreferredTheme(context)           // read

The persisted value is automatically injected as preferredTheme in globalProps for every new container. Frontend ThemeProviders can read lynx.__globalProps.preferredTheme to initialize with the correct theme.

What's NOT in this PR (follow-ups)

• Pipe method for JS to call setPreferredTheme (needs a new method in sparkling-method or debug-tool)
• Broadcasting theme changes to already-open containers (updateGlobalProps on active views)
• Respecting persisted preference in HybridContext.getTheme() when force_theme_style is absent

Test plan

• iOS: call SPKGlobalPropsUtils.setPreferredTheme("dark") → open a sub-page → verify lynx.__globalProps.preferredTheme == "dark"
• Android: call GlobalPropsUtils.setPreferredTheme(ctx, "light") → open a sub-page → verify lynx.__globalProps.preferredTheme == "light"
• Verify preferredTheme is empty string (iOS) / absent (Android) when no preference is set

[leeekyrie]

We already have a globalProps named forceThemeStyle for similar feature, not sure if that match you requirement