PAINTROID-799: Fix opacity regression after zoom and improve semi-transparent color swatch rendering

[DevAnuragT]

Short Description

This PR fixes PAINTROID-799 by resolving opacity darkening after zoom gestures and improving semi-transparent color preview rendering in the toolbar swatch.

Jira ticket: PAINTROID-799

Root Cause

After a zoom interaction, an unintended paint-end commit could still be triggered.
This affected the latest active stroke, which explains why only the most recently added semi-transparent stroke got darker.

New Features and Enhancements

• None

Refactorings and Bug Fixes

• Prevent unintended paint commit after zoom end in drawing interaction flow.
• Add stricter interaction gating so paint commit is executed only for valid single-pointer paint interactions.
• Preserve opacity stability of semi-transparent brush strokes across zoom in/out.
• Improve bottom bar color swatch rendering for semi-transparent colors using a white base layer.
• Prevent build-phase side effects from landing page database error toast handling.
• Add/adjust regression tests for:
  • opacity stability after zoom,
  • semi-transparent swatch rendering behavior,
  • updated bottom nav bar interaction helper behavior.

Validation

• flutter analyze passes on this branch.
• Relevant targeted regression test passes:
  • test/integration/brush_tool_test.dart
• Manual validation on physical device confirms the reported darkening behavior is fixed.

Video Proof

paintroid_opacity.mp4

Checklist

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[DevAnuragT]

I couldn't comment on Jira under this issue, so I thought I should open a PR directly for now