fix: 'No' button in unsaved-changes dialog does not navigate back

[mulikruchi07]

Description

Fixes the behavior of the unsaved-changes dialog shown when leaving the Task Edit screen.
Previously the No button closed the dialog without returning a value, which caused the onWillPop handler to treat it like a cancel and prevented navigation back. This change makes the No button return false from the dialog and updates onWillPop so that:

• Yes → saves then allows the route to pop,
• No → discards changes and allows the route to pop,
• Cancel → stays on the page (no pop).

Fixes #511

Screenshots

Before (issue):
(see issue #511 for the before video)

After (this PR):

after.mp4

Tapping "No" now discards changes and navigates back to the previous screen

Checklist

• Tests have been added or updated to cover the changes
• Documentation has been updated to reflect the changes
• Code follows the established coding style guidelines
• All tests are passing

[Copilot]

Pull request overview

This PR fixes a bug where the "No" button in the unsaved changes dialog on the Task Edit screen did not navigate back to the previous screen. The fix updates the dialog's return values and the onWillPop handler logic to properly distinguish between the three user actions: saving changes (Yes), discarding changes (No), and canceling the navigation (Cancel).

Key Changes

• Updated the "No" button to return false from the dialog instead of void
• Updated the "Cancel" button to return null instead of false
• Modified the onWillPop handler to allow navigation for both true (save) and false (discard) cases, while blocking navigation only for null (cancel)

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[Copilot]

[nitpick] Consider using an enum-based approach similar to taskc_details_controller.dart for better type safety and maintainability. That module defines UnsavedChangesAction { save, discard, cancel } and uses it in the dialog and handleWillPop method. This would make the intent clearer than using bool? values, especially since you now have three distinct actions (Yes/No/Cancel) rather than true/false/null.