feat: configurable heading font sizes via settings.json

[Copilot]

Heading font sizes were hardcoded in a static CSS file, making customization impossible without editing files that get clobbered on plugin updates.

Approach

• Dynamic CSS endpoint (expressCreateServer): registers GET /ep_headings2/editor.css that renders CSS from the configured font sizes at request time, replacing the static file
• Settings relay (loadSettings): reads ep_headings2.fontSizes from settings.json via ep_plugin_helpers/settings; each heading level falls back to its original default independently
• Export parity: stylesForExport now pulls from the same settings so exported documents match the editor
• aceEditorCSS: updated to point at the dynamic endpoint instead of the static CSS path

Configuration

"ep_headings2": {
  "fontSizes": {
    "h1": "2em",
    "h2": "1.6em",
    "h3": "1.3em",
    "h4": "1.1em"
  }
}

Omitted keys fall back to defaults (2.5em / 1.8em / 1.5em / 1.2em). Restart required after changing settings.