feat: Add a "Content Edit Mode" for non-technical users

### Summary

Introduce a simplified edit mode in Frappe Builder that allows non-technical users to safely update text content and images on existing pages, without exposing layout, styling, or code-editing capabilities.

### Problem

Frappe Builder is powerful but its full editor is overwhelming — and risky — for non-technical contributors (marketing, content editors, project owners). A small mistake (dragging a block, editing CSS, changing a class) can break the page layout. As a result, technical users end up handling routine content updates that should be self-service.

### Proposed solution

A toggleable **Content Edit Mode** that restricts the editor to in-place content changes only:

**Allowed in this mode:**
- Edit text content (inline, WYSIWYG-style)
- Replace images (upload / pick from library)
- Edit links (URL + label)
- Edit button labels and links

**Locked / hidden in this mode:**
- Block structure (no add / delete / move / reorder)
- Layout and spacing controls
- CSS / class / style editing
- Script editor and code blocks
- Component creation and data sources
- Page settings

### UX notes

- Toggle accessible from the top bar (e.g. "Content mode" / "Design mode")
- Visual cue: editable zones get a subtle outline on hover; everything else appears non-interactive
- Permission-based: a role (e.g. `Builder Content Editor`) could be restricted to this mode only, so non-technical users never see the full editor

### Why this matters

- Empowers non-technical teams to maintain pages independently
- Reduces accidental layout breakage
- Makes Builder viable as a CMS-like tool in organizations with mixed technical skill levels