Fix large CMS upload timeout with idle heartbeat and add per-file progress meter

Replace the 100s default HttpClient timeout (set Timeout=Infinite) with an idle/heartbeat
deadline driven by a ProgressStreamContent wrapper that reports bytes-on-the-wire. Each tick
resets the idle window and advances a MudProgressLinear per upload row. Idle window is
configurable via Upload:IdleTimeoutSeconds (default 90s).

DeepDrftManager/Services/ProgressStreamContent.cs

@@ -0,0 +1,64 @@
+using System.Net;
+
+namespace DeepDrftManager.Services;
+
+/// <summary>
+/// An <see cref="HttpContent"/> that streams a source stream to the wire while reporting cumulative
+/// bytes written after each chunk. This is the single source of truth for both the upload progress
+/// meter and the idle/heartbeat timeout: every reported tick advances the UI <em>and</em> resets the
+/// idle deadline, so one mechanism feeds both concerns.
+/// </summary>
+/// <remarks>
+/// Wrap the audio payload (not the whole multipart container) so <see cref="TryComputeLength"/>
+/// returns the file length and the reported byte counts map directly onto "bytes of this file".
+/// </remarks>
+public sealed class ProgressStreamContent : HttpContent
+{
+    // 80 KB: large enough to keep the socket fed on a healthy link, small enough that a stalled
+    // connection trips the idle window without a multi-MB write swallowing the whole heartbeat budget.
+    private const int CopyBufferSize = 81_920;
+
+    private readonly Stream _source;
+    private readonly long _length;
+    private readonly Action<long> _onBytesWritten;
+
+    /// <param name="source">The payload stream. Read once, sequentially — not seekable-rewound.</param>
+    /// <param name="length">Total bytes the source will yield; sets Content-Length and the meter denominator.</param>
+    /// <param name="onBytesWritten">Invoked after each chunk with the cumulative bytes written so far.</param>
+    public ProgressStreamContent(Stream source, long length, Action<long> onBytesWritten)
+    {
+        _source = source;
+        _length = length;
+        _onBytesWritten = onBytesWritten;
+    }
+
+    // Token-aware overload (.NET 5+): HttpClient calls this on the send path and passes the request's
+    // CancellationToken, so the idle-heartbeat CTS aborts an in-flight read/write promptly — not just
+    // between chunks. The parameterless base override delegates here with CancellationToken.None.
+    protected override Task SerializeToStreamAsync(Stream stream, TransportContext? context, CancellationToken cancellationToken)
+        => CopyAsync(stream, cancellationToken);
+
+    protected override Task SerializeToStreamAsync(Stream stream, TransportContext? context)
+        => CopyAsync(stream, CancellationToken.None);
+
+    private async Task CopyAsync(Stream stream, CancellationToken cancellationToken)
+    {
+        var buffer = new byte[CopyBufferSize];
+        long written = 0;
+        int read;
+        while ((read = await _source.ReadAsync(buffer, cancellationToken)) > 0)
+        {
+            await stream.WriteAsync(buffer.AsMemory(0, read), cancellationToken);
+            written += read;
+            // Report after the bytes are on the wire — a tick means real forward progress, which is
+            // exactly the signal the idle heartbeat must reset on.
+            _onBytesWritten(written);
+        }
+    }
+
+    protected override bool TryComputeLength(out long length)
+    {
+        length = _length;
+        return true;
+    }
+}