⚡ perf: optimize GPX export threading to prevent UI freezing

[Max97k]

💡 What:

• Modified TrackingScreen.kt and HistoryScreen.kt to wrap synchronous FileOutputStream operations in withContext(Dispatchers.IO).
• Wrapped the CPU-intensive GpxGenerator.generateGpx operations in withContext(Dispatchers.Default).

🎯 Why:
The GPX export functionality was previously executing on the main thread. Writing large files (I/O) and generating large XML strings (CPU) synchronously blocks the main UI thread, causing UI freezes, lag, and potentially Application Not Responding (ANR) crashes for users when exporting long tracking sessions.

📊 Measured Improvement:
Added GpxExportBenchmarkTest.kt to measure the execution.

• Baseline I/O time for a 10MB file on main thread: ~300ms
• Optimized I/O time (with Dispatchers.IO context switch): ~277ms

While the raw wall-clock time is similar, the critical improvement is that these operations are completely offloaded from the Main thread to Dispatchers.IO and Dispatchers.Default, freeing up the UI thread and preventing freezes.

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PR created automatically by Jules for task 9852801055414253535 started by @Max97k

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[gemini-code-assist]

Code Review

This pull request improves UI responsiveness by offloading GPX generation to Dispatchers.Default and file export operations to Dispatchers.IO in both HistoryScreen and TrackingScreen. It also introduces a benchmark test to measure GPX export performance. Feedback is provided to optimize memory usage during file writing by using a bufferedWriter instead of allocating a large byte array, and to ensure temporary files created during benchmark tests are properly cleaned up.

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[gemini-code-assist]

Converting the entire GPX string to a byte array using gpxContent.toByteArray() allocates a large byte array in memory (e.g., 10MB+ for long tracking sessions). This can cause significant memory pressure and potential OutOfMemoryError crashes on memory-constrained Android devices. Instead, write the string directly to the file using a BufferedWriter via bufferedWriter(), which encodes and writes the characters in chunks without allocating the entire byte array at once.

Suggested change

	FileOutputStream(targetFile).use {
	it.write(gpxContent.toByteArray())
	}
	targetFile.bufferedWriter().use {
	it.write(gpxContent)
	}

[gemini-code-assist]

The benchmark creates a 10MB temporary file using File.createTempFile but never deletes it. This will leak large files in the system's temporary directory every time the tests are run. Clean up the temporary file at the end of the benchmark.

        println("Note: The primary improvement is moving off the Main thread, not necessarily reducing raw I/O time. Blocking the Main thread causes UI freezes.")
        tempFile.delete()
    }