fix: allow user-defined sidebar_action to override generated fields

[djyde]

Summary

• Fixed an issue where the sidebar_action manifest field for Firefox was being completely overwritten by WXT, ignoring user-defined configuration from wxt.config.ts
• User-defined sidebar_action fields now properly override auto-generated values
• Added test case to verify the merge behavior

Changes

• Modified packages/wxt/src/core/utils/manifest.ts to spread user-defined sidebar_action configuration after generated fields, allowing user values to take precedence
• Added test case in packages/wxt/src/core/utils/__tests__/manifest.test.ts to verify user configuration is properly merged

Test Plan

• All existing tests pass (76 tests)
• New test case verifies that user-defined fields override generated values
• Custom fields (like custom_field) are preserved
• Auto-generated default_panel is still included

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