Fix macOS packaging: create .app bundle manually

wails3 build v3 alpha delegates to Taskfile; the go build produces a raw
binary, not a .app. Package step now creates the bundle structure
(Contents/MacOS, Info.plist) and zips it.

cmd/desktop/Taskfile.yml

@@ -0,0 +1,14 @@
+version: '3'
+
+tasks:
+  build:
+    summary: Build the Oikos desktop app
+    cmds:
+      - go build -o build/bin/oikos-desktop .
+    env:
+      CGO_ENABLED: 1
+
+  dev:
+    summary: Run in development mode
+    cmds:
+      - go run .