Add soft hint snackbar for missing required fields on page navigation

[najuna-brian]

Description

Adds a non-blocking snackbar hint when navigating away from a page with missing required fields.

Changes

• Shows a snackbar with missing field names when navigating forward from an incomplete page
• Includes a "Go Back" action button to return to the previous page
• Navigation remains non-blocking (users can still proceed)
• Only checks required fields when navigating forward (not backward)
• Follows Material Design 3 guidelines for snackbar styling

Behavior

• Snackbar appears at bottom center with dark background
• Displays up to 2 missing field names
• Auto-dismisses after 6 seconds
• "Go Back" button navigates back to fix missing fields
• Finalize page remains the gate for required field validation

UI changes preview

Closes #101

[r0ssing]

Love it - this is a great idea! I am wondering if it will become annoying that it sits on top of the next button though (i.e. if the user wants to blitz through some of the questions, she/he would have to wait for the toast to disappear before being able to click next again?) 🦥

[najuna-brian]

I am wondering if it will become annoying that it sits on top of the next button though (i.e. if the user wants to blitz through some of the questions, she/he would have to wait for the toast to disappear before being able to click next again?)

Yes it would be annoying, so when a user taps anywhere on the screen or swipes, it disappears though i agree may be it should appear a bit higher than the next buttons.