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/appicon.icon/Assets/wails_icon_vector.svg

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
+<svg width="100%" height="100%" viewBox="0 0 583 533" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" xml:space="preserve" xmlns:serif="http://www.serif.com/" style="fill-rule:evenodd;clip-rule:evenodd;stroke-linejoin:round;stroke-miterlimit:2;">
+    <g transform="matrix(1,0,0,1,-246,-251)">
+        <g id="Ebene1">
+            <path d="M246,251L265,784L401,784L506,450L507,450L505,784L641,784L829,251L682,251L596,567L595,567L596,251L478,251L378,568L391,251L246,251Z"/>
+        </g>
+    </g>
+</svg>

build/appicon.icon/icon.json

@@ -0,0 +1,51 @@
+{
+  "fill" : {
+    "automatic-gradient" : "extended-gray:1.00000,1.00000"
+  },
+  "groups" : [
+    {
+      "layers" : [
+        {
+          "fill-specializations" : [
+            {
+              "appearance" : "dark",
+              "value" : {
+                "solid" : "srgb:0.92143,0.92145,0.92144,1.00000"
+              }
+            },
+            {
+              "appearance" : "tinted",
+              "value" : {
+                "solid" : "srgb:0.83742,0.83744,0.83743,1.00000"
+              }
+            }
+          ],
+          "image-name" : "wails_icon_vector.svg",
+          "name" : "wails_icon_vector",
+          "position" : {
+            "scale" : 0.85,
+            "translation-in-points" : [
+              0.0,
+              0.0
+            ]
+          }
+        }
+      ],
+      "shadow" : {
+        "kind" : "neutral",
+        "opacity" : 0.5
+      },
+      "specular" : true,
+      "translucency" : {
+        "enabled" : true,
+        "value" : 0.5
+      }
+    }
+  ],
+  "supported-platforms" : {
+    "circles" : [
+      "watchOS"
+    ],
+    "squares" : "shared"
+  }
+}