feat(managers): add Flatpak package manager support

[Vishvadharman]

Description

Implemented Flatpak package manager support for TRX by adding a new FlatpakManager backend that implements the existing PackageManager trait.

Features

• Added src/managers/flatpak.rs
• Added Flatpak detection in get_available_managers()
• Added Flatpak integration in get_system_manager()
• Implemented package search using:

flatpak search <query>

• Implemented installed package listing using:

flatpak list --app --columns=application,version

• Implemented package installation using:

flatpak install -y <app-id>

• Implemented package removal using:

flatpak uninstall -y <app-id>

• Implemented update detection using:

flatpak remote-ls --updates

• Implemented package updates using:

flatpak update -y <app-id>

• Implemented full system upgrade using:

flatpak update -y

• Implemented metadata refresh support
• Added search result caching and details caching consistent with existing managers

Fixes #68

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

• Manual verification of Flatpak manager integration
• cargo test

Commands executed:

cargo fmt
cargo check

Checklist

• My code follows the Coding Guidelines.
• I have performed a self-review of my own code.
• I have NOT added useless AI-generated comments.
• I have NOT deleted existing unrelated comments.
• I have NOT modified files unrelated to this task.
• I have included screenshots/recordings (for UI changes).
• My changes generate no new warnings (clippy/fmt).
• I have linked the issue I am working on.

Screenshots (if applicable)

N/A – backend package manager implementation.

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[pie-314]

@Vishvadharman Screenshots/screen recording is applicable also you need to raise pr in dev branch

[Vishvadharman]

@pie-314 Thanks for the review. I've updated the PR to target the dev branch and added build verification screenshots. Please let me know if any further changes are needed.

[pie-314]

@Vishvadharman Your screenshot shows that, code compiles, it doesn't tell if flatpack is working. You have to upload SS or screen record.
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