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/frontend/themes/classic/theme.json

@@ -0,0 +1,8 @@
+{
+  "id": "classic",
+  "name": "Classic Cyberpunk",
+  "description": "Original dark navy aesthetic with magenta accents",
+  "supportsModes": ["dark", "light", "auto"],
+  "defaultMode": "dark",
+  "author": "Dangerous Things"
+}

app/frontend/themes/classic/tokens.css

@@ -0,0 +1,5 @@
+/*
+ * Classic Cyberpunk — Token CSS
+ *
+ * Generated by @dangerousthings/tokens during CI builds.
+ */