Load the proxy pool at startup, and stop old settings files zeroing new defaults

The pool was empty until someone pressed Refresh, which also meant the parser
would have had nothing to work with. A ProxyPoolLoader now fills it once at
launch; startup does not await it, because blocking on a public list being
reachable would be the wrong trade, and it never throws. The proxy page joins
the same operation rather than starting a second download, so it reports the
outcome whether it is opened during the load or long after.

That change surfaced a worse bug underneath. The first run still loaded zero
entries with no error logged at all, which turned out to be the feed source
never being asked: options said UseFeed=False and Protocols=None. Neither is
reachable from the UI — both are default(T).

The cause is that AppSettings kept its defaults on property initialisers, and
the source-generated deserialiser does not run them. Reflection-based
deserialisation of "{}" keeps them; the generated context does not. So a
settings.json written before a setting existed came back with default(T) for it:
the proxy feed switched off, the protocol filter empty, the probe timeout zero
and the probe URL blank — and the app looked like the network had failed.

Defaults now live on primary constructor parameters, which STJ applies for
absent JSON members on both paths, so an older file upgrades cleanly. The
regression test writes a settings file from before the proxy settings existed
and asserts each one comes back at its default. ToProxyOptions also treats an
empty protocol filter as "all", since a hand-edited file that matches nothing is
the least useful possible reading of it.

Also quietens IHttpClientFactory to Warning: four Information lines per request
buried everything the app said, and a proxy sweep makes thousands of them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/AvParser.Core/Settings/AppSettings.cs

@@ -19,57 +19,54 @@ public enum AppTheme
 /// Everything the app remembers between runs. Persisted verbatim as JSON.
 /// </summary>
 /// <remarks>
-/// A mutable record with defaults on every property: a settings file written by an older
-/// version must still deserialise, so no property may be required.
+/// <para>
+/// Defaults live on the <b>primary constructor parameters</b>, not on property initialisers.
+/// That is not a style choice: the source-generated serialiser constructs the object without
+/// running property initialisers, so a settings file written by an older build came back with
+/// <c>default(T)</c> for every setting added since — which silently switched the proxy feed off
+/// and zeroed the probe timeout. Constructor parameter defaults are applied for absent JSON
+/// members, so an old file now upgrades cleanly.
+/// </para>
+/// <para>
+/// No property may be required, for the same reason: an old file must still deserialise.
+/// </para>
 /// </remarks>
-public sealed record AppSettings
+/// <param name="Theme">Chosen theme variant.</param>
+/// <param name="Language">Chosen UI language.</param>
+/// <param name="LastParserId">Id of the parser selected last time; resolved leniently on load.</param>
+/// <param name="WindowWidth">Last main-window width in device-independent pixels.</param>
+/// <param name="WindowHeight">Last main-window height in device-independent pixels.</param>
+/// <param name="WindowMaximized">Whether the main window was maximised on exit.</param>
+/// <param name="MinimumLogLevel">Minimum Serilog level, as a Serilog level name.</param>
+/// <param name="ProxyRotation">How the pool picks the next proxy.</param>
+/// <param name="ProxyHealthCheck">When proxy liveness is verified.</param>
+/// <param name="ProxyUseFeed">Whether the remote proxy feed is consulted.</param>
+/// <param name="ProxyProtocols">Protocols accepted when loading proxy sources.</param>
+/// <param name="ProxyProbeUrl">URL fetched to decide whether a proxy works.</param>
+/// <param name="ProxyProbeTimeoutSeconds">Per-proxy probe timeout, in seconds.</param>
+/// <param name="ProxyProbeConcurrency">How many probes run at once during a pool sweep.</param>
+public sealed record AppSettings(
+    AppTheme Theme = AppTheme.System,
+    AppLanguage Language = AppLanguage.System,
+    string? LastParserId = null,
+    double WindowWidth = 1280,
+    double WindowHeight = 800,
+    bool WindowMaximized = false,
+    string MinimumLogLevel = "Information",
+    ProxyRotation ProxyRotation = ProxyRotation.Sticky,
+    ProxyHealthCheck ProxyHealthCheck = ProxyHealthCheck.Pool,
+    bool ProxyUseFeed = true,
+    ProxyProtocolFilter ProxyProtocols = ProxyProtocolFilter.All,
+    string ProxyProbeUrl = "http://www.gstatic.com/generate_204",
+    int ProxyProbeTimeoutSeconds = 8,
+    int ProxyProbeConcurrency = 64
+)
 {
-    /// <summary>Chosen theme variant.</summary>
-    public AppTheme Theme { get; init; } = AppTheme.System;
-
-    /// <summary>Chosen UI language.</summary>
-    public AppLanguage Language { get; init; } = AppLanguage.System;
-
-    /// <summary>Id of the parser selected last time; resolved leniently on load.</summary>
-    public string? LastParserId { get; init; }
-
-    /// <summary>Last main-window width in device-independent pixels.</summary>
-    public double WindowWidth { get; init; } = 1280;
-
-    /// <summary>Last main-window height in device-independent pixels.</summary>
-    public double WindowHeight { get; init; } = 800;
-
-    /// <summary>Whether the main window was maximised on exit.</summary>
-    public bool WindowMaximized { get; init; }
-
-    /// <summary>Minimum Serilog level, as a Serilog level name.</summary>
-    public string MinimumLogLevel { get; init; } = "Information";
-
-    /// <summary>How the pool picks the next proxy.</summary>
-    public ProxyRotation ProxyRotation { get; init; } = ProxyRotation.Sticky;
-
-    /// <summary>When proxy liveness is verified.</summary>
-    public ProxyHealthCheck ProxyHealthCheck { get; init; } = ProxyHealthCheck.Pool;
-
-    /// <summary>Whether the remote proxy feed is consulted.</summary>
-    public bool ProxyUseFeed { get; init; } = true;
-
-    /// <summary>Protocols accepted when loading proxy sources.</summary>
-    public ProxyProtocolFilter ProxyProtocols { get; init; } = ProxyProtocolFilter.All;
-
-    /// <summary>URL fetched to decide whether a proxy works.</summary>
-    public string ProxyProbeUrl { get; init; } = "http://www.gstatic.com/generate_204";
-
-    /// <summary>Per-proxy probe timeout, in seconds.</summary>
-    public int ProxyProbeTimeoutSeconds { get; init; } = 8;
-
-    /// <summary>How many probes run at once during a pool sweep.</summary>
-    public int ProxyProbeConcurrency { get; init; } = 64;
-
     /// <summary>Projects the proxy-related settings onto <see cref="ProxyOptions"/>.</summary>
     /// <remarks>
     /// Settings are persisted as primitives so an old file still deserialises; the pool wants a
-    /// validated options object. This is the single place that bridges the two.
+    /// validated options object. This is the single place that bridges the two, and it clamps
+    /// rather than throws so a hand-edited file cannot stop the app from starting.
     /// </remarks>
     public ProxyOptions ToProxyOptions()
     {
@@ -82,7 +79,7 @@ public sealed record AppSettings
             Rotation = ProxyRotation,
             HealthCheck = ProxyHealthCheck,
             UseFeed = ProxyUseFeed,
-            Protocols = ProxyProtocols,
+            Protocols = ProxyProtocols == ProxyProtocolFilter.None ? ProxyProtocolFilter.All : ProxyProtocols,
             ProbeUrl = probeUrl,
             ProbeTimeout = TimeSpan.FromSeconds(Math.Clamp(ProxyProbeTimeoutSeconds, 1, 120)),
             ProbeConcurrency = Math.Clamp(ProxyProbeConcurrency, 1, 512),