Add saved templates library with iframe thumbnail previews

[sagnik11]

TL;DR

Implements a saved templates library feature allowing users to save campaigns as reusable templates, browse them with live thumbnail previews, and start new campaigns from any template. Updates documentation and marks the feature as complete in the roadmap.

What changed?

• Documentation updates:

  • Added saved templates library feature to CHANGELOG.md
  • Added feature description to README.md highlighting template saving, browsing with thumbnails, and campaign creation from templates
  • Marked "Saved templates library" as completed in ROADMAP.md

• Editor UI enhancements:

  • Added "Save as Template" button to campaign editor toolbar
  • Implemented save template dialog with fields for template name and optional description
  • Added state management for template saving flow (dialog open/close, form inputs, loading state)

• Template saving logic:

  • Implemented handleSaveAsTemplate() function that:
    • Validates template name is provided
    • Saves current draft before creating template snapshot
    • Makes POST request to /api/internal/campaigns/{campaignId}/save-as-template
    • Shows success/error toast notifications
  • Snapshots current subject, sender info, and email content into the template

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