fix: expose divider alignment in style manager

[sridharkalaibala]

Selecting an mj-divider currently hides the existing Align control because align is missing from its stylable properties. Enable it and set the divider's default alignment to center, matching MJML's documented default.

Closes #390.

This uses the existing Style Manager control and style-to-attribute conversion. Explicit left/right alignment is preserved when importing MJML. The center default is omitted from exported MJML through the existing default-attribute handling.

Validation:

• Seven tests cover control visibility, center selection/default export, left/center/right changes through Style Manager and compiled HTML, and explicitly imported left/right values.
• Original source: 2 failures and 5 passing controls. Fixed source: all 7 pass.
• Full suite: 26 tests and 2 existing snapshots pass on Node 24.19.0 and CI's Node 18 family (18.20.8).
• npm run lint, npx tsc --noEmit, and npm run build pass. Build produces the library, locale files and TypeScript declarations; existing bundle-size/Browserslist warnings remain.
• Chromium using the same built plugin with both locked GrapesJS 0.21.2 and published GrapesJS 0.23.6: center is initially selected; clicking each of left/right/center updates the model, actual divider position in the canvas, and compiled HTML. No runtime errors.

No dependency, lockfile or version changes. AI assistance: implemented and validated with OpenAI Codex.