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

Taskfile.yml

@@ -0,0 +1,63 @@
+version: '3'
+
+vars:
+  APP_NAME: "yellowjacket"
+  BIN_DIR: "bin"
+  PACKAGE_MANAGER: '{{.PACKAGE_MANAGER | default "pnpm"}}'
+  VITE_PORT: '{{.WAILS_VITE_PORT | default 9245}}'
+  # Target OS for build/package/run. Defaults to the host OS, and is overridden
+  # by `wails3 build GOOS=...` (or the GOOS env var) for cross-compilation. The
+  # tasks below dispatch to the matching platform Taskfile via this variable.
+  GOOS: '{{.GOOS | default OS}}'
+
+includes:
+  common: ./build/Taskfile.yml
+  windows: ./build/windows/Taskfile.yml
+  darwin: ./build/darwin/Taskfile.yml
+  linux: ./build/linux/Taskfile.yml
+
+tasks:
+  build:
+    summary: Builds the application
+    cmds:
+      - task: "{{.GOOS}}:build"
+
+  package:
+    summary: Packages a production build of the application
+    cmds:
+      - task: "{{.GOOS}}:package"
+
+  run:
+    summary: Runs the application
+    cmds:
+      - task: "{{.GOOS}}:run"
+
+  dev:
+    summary: Runs the application in development mode
+    cmds:
+      - wails3 dev -config ./build/config.yml -port {{.VITE_PORT}}
+
+  setup:docker:
+    summary: Builds Docker image for cross-compilation (~800MB download)
+    cmds:
+      - task: common:setup:docker
+
+  build:server:
+    summary: Builds the application in server mode (no GUI, HTTP server only)
+    cmds:
+      - task: common:build:server
+
+  run:server:
+    summary: Runs the application in server mode
+    cmds:
+      - task: common:run:server
+
+  build:docker:
+    summary: Builds a Docker image for server mode deployment
+    cmds:
+      - task: common:build:docker
+
+  run:docker:
+    summary: Builds and runs the Docker image
+    cmds:
+      - task: common:run:docker