fix: accessibility improvements in Post Editor

[collinsezedike]

Description

The Post Editor had a few real accessibility gaps: embedded images shipped with no alt text, the template sidebar's heading text didn't match its expected accessible name, and the publish action gave screen-reader users no feedback while submitting.

Changes

• Add an alt-text field to the image embed dialog (MediaEmbedder) and thread it through useContentEditor's addImage so embedded images ship with proper alt text (WCAG 1.1.1)
• Update the template sidebar heading from "Templates" to "Content Templates" so its accessible name matches what screen readers announce
• Add aria-busy and a polite aria-live status region to MaterialEditorWrapper's publish button so screen-reader users are notified while a submit is in progress

Testing

• Extended src/components/editor/__tests__/accessibility.test.tsx to cover the new alt-text field (including that it only shows for image embeds) and the publish status announcement

Closes #364

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