feat(shell): create application shell and navigation architecture

[ibourgeois]

Goal

Create the shared 3-column application shell that future apps will live inside.

Scope

• Build the desktop-first Atlassian-style layout
• Left toolbar for application selection
• Middle context navigation region
• Main content region
• Define responsive collapse behavior

Out of Scope

• Final theming system
• Auth flows
• Specific ideas or admin business features

Acceptance Criteria

• Authenticated pages can render inside a reusable shell
• The shell exposes a stable spot for toolbar, context nav, and main content
• The layout is usable on desktop and degrades sensibly on smaller screens

Dependencies

• Depends on chore(frontend): install and configure Livewire and icon package #3

Test Plan

• Add focused component or feature coverage for shell rendering
• Manually verify layout behavior across desktop and mobile breakpoints