fix(flatpak): Add Slack URL scheme support for OAuth flow

[jkuro-tii]

• Implement slack:// URL handling within the Flatpak VM
• Ensure Slack launches correctly with authorization links

Slack OAuth login now works end-to-end, allowing authorization links to properly open inside the Flatpak sandbox instead of failing or looping.

Description of Changes

Type of Change

• New Feature
• Bug Fix
• Improvement / Refactor

Related Issues / Tickets

Checklist

• Clear summary in PR description
• Detailed and meaningful commit message(s)
• Commits are logically organized and squashed if appropriate
• Contribution guidelines followed
• Ghaf documentation updated with the commit - https://tiiuae.github.io/ghaf/
• Author has run make-checks and it passes
• All automatic GitHub Action checks pass - see actions
• Author has added reviewers and removed PR draft status

Testing Instructions

Applicable Targets

• Orin AGX aarch64
• Orin NX aarch64
• Lenovo X1 x86_64
• Dell Latitude x86_64
• System 76 x86_64

Installation Method

• Requires full re-installation
• Can be updated with nixos-rebuild ... switch
• Other:

Test Steps To Verify:

1. Open Applications → App Store in the Flatpak VM.
2. Install Slack and at least one web browser (e.g., Chrome/Firefox/Brave/Chromium).
3. Launch Slack.
4. Sign in using the "Sign in with browser" flow.
5. Verify that:
   • The browser opens the Slack authorization page.
   • Slack completes the sign-in process and reaches the workspace.
6. Sign out and sign back in again to confirm the OAuth redirect works reliably.

[vunnyso]

I have a design question regarding this PR. Do we need to support Slack through the App Store, considering we already have Slack in comms-vm? IMO, supporting the same apps in different VMs could potentially cause user confusion and pose a security risk.