feat(import-export): support user-created strategy backup and restore

[dhruvanbhalara]

Description

This PR implements support for exporting and importing user-created password generation strategies under a unified vault backup format. Custom strategies are now packaged alongside credentials in JSON and encrypted .pvault formats, and correctly resolved/restored during vault restoration.

Key changes:

• Refactored DataService to format JSON and encrypted backups into a unified map format: { "passwords": [...], "strategies": [...] }.
• Integrated backward compatibility to auto-detect and parse legacy password-only files.
• Refactored ImportExportBloc and ImportExportPathResolver to handle backup generation and restore for custom strategies, incorporating a conflict resolution strategy (skips default strategies, overwrites matching IDs, and resolves name collisions).
• Added a dynamic warning banner in the ExportVaultScreen when the CSV format option is selected since strategies cannot be represented in tabular CSV format.
• Dispatched LoadSettings() to SettingsBloc upon successful import or duplicate resolution to update the in-memory strategy configuration.

Related Issues

Closes #47

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🐛 Bug fix (non-breaking change which fixes an issue)
• 🎨 UI/UX refinement

Checklist

• I have followed the project's CONTRIBUTING.md guidelines.
• My code follows the style guidelines of this project.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.

Screenshots (if applicable)

N/A

Testing Instructions

1. Navigate to Settings -> Password Generation Strategies, and create a custom strategy.
2. Go to Settings -> Export Vault, enter a password, and export as Encrypted (.pvault).
3. Confirm that when selecting CSV format, a warning banner is shown dynamically stating that CSV does not include strategies.
4. Clear the database (or delete the custom strategy).
5. Go to Settings -> Import Data, select the exported .pvault, enter the password, and resolve conflicts.
6. Verify that the custom strategy is successfully imported and displayed in the Strategy Screen.

📊 Code Coverage Report

Metric	Value
Total Lines	5287
Lines Hit	4285
Overall Coverage	81.05%

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