Fix file reopening after exit confirmation on Android

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Description

Fixes #40 - Files opened via Android Intent persist in MainActivity memory after exit confirmation, causing them to reopen when launching from recent apps.

Root cause: launchMode="singleTop" prevents Activity destruction on exit. fileContentToSend remains populated and gets sent again on app restore.

Changes:

• Android (MainActivity.kt): Added clearFileContent method channel handler to null out stored file content
• Flutter (home.dart): Call clearFileContent before SystemNavigator.pop() in exit confirmation handler

// MainActivity.kt
when (call.method) {
    "getFileContent" -> {
        result.success(fileContentToSend)
        fileContentToSend = null
    }
    "clearFileContent" -> {
        fileContentToSend = null  // Clear on explicit exit
        result.success(null)
    }
    else -> result.notImplemented()
}

// home.dart
final bool shouldPop = await _showExitConfirmationDialog();
if (shouldPop && context.mounted) {
    await _clearFileContent();  // Clear before exit
    await SystemNavigator.pop(animated: true);
}

Result: Clean app state on restore, no stale file content.

Original prompt

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This section details on the original issue you should resolve

<issue_title>after closing a file (resp. the app and confirming) the file get's reopened by itself</issue_title>
<issue_description>Describe the bug
i found a somewhat odd behavior... so this is more or less a follow up on #18:
at least on Android since version 2.0.0 Markdown Editor asks every time you want to exit it via back button - also if you haven't changed anything or even if there is nothing in the input field - for confirmation... which is IMHO unnecessary for these 2 cases, but ok.
the issue is, after confirming - and hence agreeing to close the file and lose all (unsaved) edits, exact the same file opens again if you open Markdown-Editor from the history.

to rule out a possible misunderstanding: i'm not talking about the situation when the app was just put in the background via home button or the like! - in this situation it doesn't asks for confirmation and keeps the edits, which is fine and expected. 👍

To Reproduce
Steps to reproduce the behavior:

1. open a file via file browser in Markdown Editor
   (interestingly if i open Markdown Editor first and the file via its menu, is seems to work more or less as expected as also an empty Markdown Editor gets opened afterwards via the app history.)
2. make some - minor - edits (perhaps at the end or somewhere in-between of a long file)
3. notice before saving that some of the edits were wrong and you would rather restart your edits to be sure that nothing wrong gets saved... hence exit Markdown Editor (without saving) via back button and confirming it.
4. notice that Markdown Editor is still in the app history and to spare some time use that to open the app.
5. be surprised that without adding anything, the file is also again/still(?) open.
6. be unsure, if the edits - you don't want to keep - are still there or not... 😒

Expected behavior
after confirming to exit, i would expect at least the file to be really closed... as i don't want to be greeted with a file where i can't be sure if it still contains - potentially unsaved but discarded - edits, which i have to check first.
perhaps even better would be to really exit/end the app and don't show it in the history as some other apps with a explicit exit button behave (eg. Firefox), as this could also be a privacy issue in some circumstances as you can still see the previously opened file in the history (even after you confirmed to exit the app).

Smartphone (please complete the following information):

• Device: Sony Xperia XA2
• OS: Android 15 resp. LineageOS 22.2
• Version 2.0.0 (8)
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes after closing a file (resp. the app and confirming) the file get's reopened by itself #43

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