Replace direct file access with Storage Access Framework for markdown export

[Copilot]

The export feature saved files to a hardcoded system directory (/storage/emulated/0/Documents/VoiceNotes/), showed truncated paths in Toast messages, and provided no way to choose save location.

Changes

MainActivity.kt

• Replace direct file writing with ACTION_CREATE_DOCUMENT intent
• Add writeNotesToUri() to handle ContentResolver-based writing
• Optimize file writing with single write operation using buildString
• Add explicit null check on openOutputStream() with error feedback

// Before: Direct file access
val file = File(documentsDir, "VoiceNotes/$filename")
FileWriter(file).use { writer ->
    writer.write("# Voice Notes Export\n\n")
    // ... multiple writes
}

// After: SAF with user-chosen location
val intent = Intent(Intent.ACTION_CREATE_DOCUMENT).apply {
    type = "text/markdown"
    putExtra(Intent.EXTRA_TITLE, "voice-notes-$timestamp.md")
}
startActivityForResult(intent, CREATE_FILE_REQUEST_CODE)

// Write once with buildString
contentResolver.openOutputStream(uri)?.use { 
    it.write(markdownContent.toByteArray())
} ?: throw Exception("Failed to open output stream")

AndroidManifest.xml

• Remove WRITE_EXTERNAL_STORAGE permission (unnecessary with SAF)

strings.xml

• Update success message to remove file path reference

Users now get a system file picker to choose save location, with a pre-filled ISO 8601 timestamped filename. No storage permissions required.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fix Export to Markdown feature</issue_title>
<issue_description>Current behavior:

• A click in the "Export to markdown" button yields a "Notes exported successfully to: /storage/emulated/0/Dcouments..." and nothing else.

Problems:

• I couldn't see the entire file path.
• It was very hard for me to find the file in the "File Explorer". My phone is in Spanish, and turns out they are under "/Almacenamiento Interno/Documents/VoiceNotes/voice-notes-20260101T18-19-45.md".
• I didn't get prompted for a custom storage location, which I was expecting.

</issue_description>

<agent_instructions>Could you improve the overall experience taking into account this issue and the original #6?</agent_instructions>

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

• Fixes Fix Export to Markdown feature #12

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