First-class Dashboards view for all users

### Description

Make the Dashboards sidebar item/view available to all users (not just dashboard-only viewers) and elevate dashboards to a first-class citizen in FlowFuse.

### User Story

As a FlowFuse user, I want to see and access all my dashboards from a dedicated view so that I can quickly open them without navigating through instances, and use FlowFuse Expert in the context of my dashboards.

### Problem

Currently:
- The Dashboards sidebar item is only available to dashboard-only viewers
- It functions more as an instance overview list than a true dashboard view
- Users must navigate to instances to find and open dashboards
- FlowFuse Expert is available to all users, but not in the context of viewing a dashboard

### Proposed UX

List/detail pattern similar to email inboxes:
- **Left panel**: List of all dashboards with their titles
- **Right panel**: Selected dashboard opens inline

Users can select a dashboard from the list and view it alongside the list, or expand to full screen.

### Benefits

- **First-class dashboards**: Dashboards become a primary navigation concept, not buried under instances
- **Expert in dashboard context**: Users can access FlowFuse Expert while viewing their dashboard - enabling questions about dashboard data, troubleshooting, or guidance
- **Improved discoverability**: Users can quickly see and access all dashboards across their instances

### Epic/Story

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate