Fix config merge order to respect user config when input signals are undefined

[Copilot]

Addresses feedback on PR #1: input signals were falling back to hardcoded defaults instead of user-provided config values when undefined.

Change
Modified the config merge in NgxMarkdownEditorComponent constructor to:

1. Merge DEFAULT_EDITOR_CONFIG with user config() into baseConfig
2. Apply input signals with baseConfig as fallback instead of hardcoded values

Before:

const mergedConfig = {
  ...DEFAULT_EDITOR_CONFIG,
  ...this.config(),
  showPreview: this.showPreviewInput() ?? true,  // Hardcoded fallback
};

After:

const baseConfig = { ...DEFAULT_EDITOR_CONFIG, ...this.config() };
const mergedConfig = {
  ...baseConfig,
  showPreview: this.showPreviewInput() ?? baseConfig.showPreview,  // Respects user config
};

This ensures precedence: defaults < user config < explicit input signals.

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