build(wails): add the v3 toolchain alongside v2

Phase 1 of plan 009. The app still builds and runs on v2 — nothing in
main.go or backend/ has moved yet — but the v3 CLI, its pinned runtime
and its build-asset tree are now present, which is what Phase 2 needs.

- wails/v3 v3.0.0-beta.8 pinned per D5, and wails3 added to the `tool`
  block beside the v2 CLI per D3. Both are vendored; neither is a
  global install.
- build/ now holds the v3 build assets, copied wholesale from a
  `wails3 init` scaffold rather than hand-written. That collides with
  this repo's existing use of build/ as ignored build *output*, so
  .gitignore narrows to build/bin/ and bin/ and the assets are tracked.
  The mobile platforms are not carried: this is a desktop player
  (MPRIS over D-Bus, beep, XDG paths) and cannot target them.
- build/config.yml's info block is filled from wails.json, which stays
  for now because the v2 CLI still reads it.
- Taskfile.yml defaults PACKAGE_MANAGER to pnpm, since the scaffold
  assumes npm and frontend/package.json.md5 is part of the dep-caching
  scheme.

One deviation from the plan worth recording. webkitgtk-6.0 is not
installed on this machine, so the default GTK4 path is unavailable and
the gtk3 fallback is in use. That also means `go tool wails3` does not
work — the CLI itself fails to compile without webkitgtk-6.0 — while
`go run -tags gtk3 .../cmd/wails3` does. The Makefile rewrite in the
next commit has to account for that, and it goes away once the GTK4
dependency is installed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UDCbcCZQepnpSQYJ6SxxZm

build/windows/msix/template.xml

@@ -0,0 +1,54 @@
+<?xml version="1.0" encoding="utf-8"?>
+<MsixPackagingToolTemplate
+    xmlns="http://schemas.microsoft.com/msix/packaging/msixpackagingtool/template/2022">
+    <Settings
+        AllowTelemetry="false"
+        ApplyACLsToPackageFiles="true"
+        GenerateCommandLineFile="true"
+        AllowPromptForPassword="false">
+    </Settings>
+    <Installer
+        Path="yjref"
+        Arguments=""
+        InstallLocation="C:\Program Files\My Company\My Product">
+    </Installer>
+    <PackageInformation
+        PackageName="My Product"
+        PackageDisplayName="My Product"
+        PublisherName="CN=My Company"
+        PublisherDisplayName="My Company"
+        Version="0.1.0.0"
+        PackageDescription="A yjref application">
+        <Capabilities>
+            <Capability Name="runFullTrust" />
+            
+        </Capabilities>
+        <Applications>
+            <Application
+                Id="com.example.yjref"
+                Description="A yjref application"
+                DisplayName="My Product"
+                ExecutableName="yjref"
+                EntryPoint="Windows.FullTrustApplication">
+                
+            </Application>
+        </Applications>
+        <Resources>
+            <Resource Language="en-us" />
+        </Resources>
+        <Dependencies>
+            <TargetDeviceFamily Name="Windows.Desktop" MinVersion="10.0.17763.0" MaxVersionTested="10.0.19041.0" />
+        </Dependencies>
+        <Properties>
+            <Framework>false</Framework>
+            <DisplayName>My Product</DisplayName>
+            <PublisherDisplayName>My Company</PublisherDisplayName>
+            <Description>A yjref application</Description>
+            <Logo>Assets\AppIcon.png</Logo>
+        </Properties>
+    </PackageInformation>
+    <SaveLocation PackagePath="yjref.msix" />
+    <PackageIntegrity>
+        <CertificatePath></CertificatePath>
+    </PackageIntegrity>
+</MsixPackagingToolTemplate>