Sidepanel and email sync fixes

[nidh-eesh]

This pull request makes several improvements and simplifications to the frontend layout and navigation logic, primarily focusing on removing the Dashboard page and streamlining how fields are handled in the Side Panel components. The changes enhance robustness when working with dynamic field definitions and simplify the user flow to always default to the Leads page.

Navigation and Routing Updates:

• Removed the Dashboard page and its related navigation and route definitions, so users are now always redirected to the Leads page by default. (frontend/src/router.js, frontend/src/components/Layouts/AppSidebar.vue) [1] [2] [3] [4]

Side Panel Field Handling Improvements:

• Updated logic in SidePanelLayout.vue and SidePanelModal.vue to robustly handle fields that may be defined as either full objects or simple string fieldnames, ensuring consistent rendering and saving of fields. (frontend/src/components/SidePanelLayout.vue, frontend/src/components/Modals/SidePanelModal.vue) [1] [2] [3]
• Improved display of "Table MultiSelect" fields by adding a utility function to render their values more flexibly, accommodating both array and JSON string formats. (frontend/src/components/SidePanelLayout.vue) [1] [2]

Field Type Restrictions:

• Removed "Table MultiSelect" from the list of restricted field types in the Side Panel Layout Editor, allowing it to be used in side panels. (frontend/src/components/SidePanelLayoutEditor.vue)

Minor Codebase Cleanups:

• Added missing import for frappe in crm/overrides/email_receive.py for completeness.
• Improved key handling in field rendering to better support both string and object field definitions. (frontend/src/components/SidePanelLayout.vue)

Edit:
Added function to check if private/files folder exists and create it if not. This is required as syncing email attachments is dependent on private/files folder being present