feat/ai-theme-generator

[Aryan-Verma-999]

feat(layout-editor): add developer AI theme generator

Acceptance Criteria fulfillment

• Add a developer-only AI Theme Generator to the Layout Editor with direct local Ollama support, optional OpenAI/Gemini adapters, and an offline deterministic fallback. No proxy is required.
• Support theme generation, focused follow-up refinements, undo, palette editing, typography, corner shape, message view, display-name variants, apply/reset, and JSON copy/download.
• Generate accessible light and dark theme schemes, preserve readable chat metadata and controls, and document local setup and CORS configuration.
• Reuses existing Layout Editor color-picker and configuration options

Video/Screenshots

2026-08-02.15-52-43.2.mp4

PR Test Details

Note: The PR will be ready for live testing at https://rocketchat.github.io/EmbeddedChat/pulls/pr-<pr_number> after approval. Contributors are requested to replace <pr_number> with the actual PR number.