Build optimization: pre-built PM3 binaries, ARM64 CI, base image caching

Replace PM3 compile-from-source in pi-gen with pre-built tarball extraction
(saves 43-58 min). Merge stagePM3 into stageDangerousPi as 02-pm3-install
substage, renumber all subsequent substages. Switch CI PM3 build to native
ARM64 runner (ubuntu-24.04-arm64) eliminating QEMU overhead. Add weekly
base-image workflow for pre-baking stages 0-2. Support PM3_TARBALL,
BASE_IMAGE, and APT_PROXY env vars in build-image.sh.

Also includes prior Phase 5 work: theme system, design system integration,
component update system, OS updates, CI build pipeline, and test results.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/plugins/supra_theme/plugin.json

@@ -0,0 +1,10 @@
+{
+  "id": "supra_theme",
+  "name": "VivoKey Supra Theme",
+  "version": "1.0.0",
+  "description": "Material Design 3 theme with VivoKey blue, rounded corners, and elevation shadows",
+  "author": "Dangerous Things",
+  "homepage": "https://github.com/dangerous-tac0s/dt-design-system",
+  "dependencies": [],
+  "permissions": []
+}