Introduce desktop shell layout for web dashboard

[dhruvi-16-me]

Summary

This PR introduces the initial desktop shell foundation for Flutter Web.

The implementation establishes a responsive desktop layout inspired by the current Figma designs while preserving all existing mobile behavior.
FIGMA LINK - https://www.figma.com/design/8S4AEiGfj4vwSOpxvBAk7E/Ell-ena-Designs-1?node-id=0-1&t=GPZKr6tUBUZLrgQT-1

The goal of this PR is to provide a reusable desktop shell that can host existing application screens without requiring desktop-specific screen rewrites.

Screenshots

BEFORE

AFTER

Changes

Desktop Shell Foundation

Added:

• AppShell
• DesktopShell
• ShellNavigationRail
• ShellDestination

Desktop layouts now render inside a shared shell structure:

Responsive Layout

• Added desktop breakpoint handling.
• Desktop users see the new shell layout.
• Mobile users continue to use the existing application layout.

Screen Reuse

Existing screens are reused directly within the desktop shell.

No desktop-specific versions of the following screens were introduced:

• DashboardScreen
• CalendarScreen
• WorkspaceScreen
• ChatScreen
• ProfileScreen

Follow-up Work

Future PRs will focus on:

1. NavigationRail tab switching.
2. go_router integration.
3. Deep linking and browser history support.
4. Screen-specific desktop optimizations.
5. Responsive improvements across Dashboard, Calendar, Workspace, Chat, and Profile.

✅ Checklist

• I have read the contributing guidelines.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if applicable).
• Any dependent changes have been merged and published in downstream modules.

Summary by CodeRabbit

Release Notes

• New Features
  • Added responsive app shell that automatically adapts between desktop and mobile layouts.
  • Introduced desktop navigation sidebar with quick access to all application destinations.
  • Implemented automatic layout switching at 1024px screen width breakpoint.

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