Meta sync: pull the club's metas and navs into the shared profiles

The original kept VTank's profile folder in step with the club's metas
repository; the port does the same against host.VtankProfiles, dropping
.met files under mosstank/metas and .nav files under mosstank/navs where
MossTank reads dropped profiles, flattened to their bare names as the
original flattened them. The Gitea tree and raw endpoints are the
original's; the repository address is the metas_repository_url setting,
defaulting to the original repository. An overwritten file keeps its old
copy as .bak (MossTank's pickers skip .bak side files).

Storage is text-only, so the SHA-256 compare hashes the text a stored read
gives back (UTF-8, BOM dropped); for the plain-text files in the repo that
equals the original's byte compare. Network work runs on the thread pool,
one operation after another; chat lines and window updates are applied on
the tick. The four /mm verbs keep their chat lines, and the Metas tab is
its own markup window with IsVisible for the main window to open.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/OpenAC.MosswartMassacre.Tests/Ui/TrackerAndMetasMarkupContractTests.cs

@@ -1,6 +1,7 @@
 using System.Reflection;
 using System.Xml.Linq;
 using OpenAC.MosswartMassacre.FlagTracker;
+using OpenAC.MosswartMassacre.Metas;
 
 namespace OpenAC.MosswartMassacre.Tests.Ui;
 
@@ -19,6 +20,7 @@ public sealed class TrackerAndMetasMarkupContractTests
     public static TheoryData<string, Type> MarkupFiles => new()
     {
         { FlagTrackerFeature.MarkupFile, typeof(FlagTrackerViewModel) },
+        { MetaSyncFeature.MarkupFile, typeof(MetasViewModel) },
     };
 
     [Theory]