[PAINTROID - 776] Second instance of app is opened when catrobat image is opened from files

[this-is-varunxyz]

Jira Ticket: PAINTROID-776
Refactorings and Bug Fixes
Summary of Changes

This PR resolves the issue where opening an image from an external file manager (via "Open with") would launch a duplicate instance of the app. It also implements the necessary native bridge to read and load the image data from secure Android URIs.
Technical Details

AndroidManifest.xml: Changed launchMode to singleTask. This ensures that if the app is already running, a new intent brings the existing task to the front instead of spawning a new window.

MainActivity.kt:

    Overrode onNewIntent to capture file intents when the app is resumed from the background.

    Extended the file_handler MethodChannel with a getFileBytes method. This uses ContentResolver to read secure content:// URIs, which standard Dart file I/O cannot access.

Flutter Implementation:

    main.dart: Now checks for an initial file URI immediately at startup and passes it to the App widget.

    landing_page.dart: Added a lifecycle listener. When the app resumes, it checks for a new file URI, fetches the image bytes via the native bridge, decodes the image, and loads it directly onto the canvas.

Checklist
Your checklist for this pull request

[x] Include the name of the Jira ticket in the PR’s title

[x] Add the link to the ticket in Jira in the description of the PR

[x] Include a summary of the changes plus the relevant context

[x] Choose the proper base branch (develop)

[x] Confirm that the changes follow the project’s coding guidelines (Wiki)

[x] Verify that the changes generate no compiler or linter warnings

[x] Perform a self-review of the changes

[x] Verify to commit no other files than the intentionally changed ones

[x] Include reasonable and readable tests verifying the added or changed behavior (Verified manually via "Open with" scenarios on Android)

[x] Confirm that new and existing tests pass locally

[x] Check that the commits’ message style matches the [project’s guideline](https://github.com/Catrobat/Catroid/wiki/Commit-Message-Guidelines)

[x] Verify that your changes do not have any conflicts with the base branch

[ ] After the PR, verify that all CI checks have passed

[ ] Add new information to the [Wiki](https://github.com/Catrobat/Paintroid-Flutter/wiki) (N/A for this bug fix)