ci: run development, quality checks and CI on Bun 1.4.3

Pin Bun 1.4.3 with packageManager, and force every bun run script onto
the Bun runtime through bunfig so ng, ESLint and Prettier never fall
back to an installed Node. Type checking uses bun check for the
application and test projects, next to the Angular build.

CI jobs install the pinned release from a checksum-verified download
instead of npm and run project steps in a shell without Node on PATH,
which stays available only for the checkout and upload actions. The
image test's fake docker runs on Bun.

Refs #290, #293

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

bunfig.toml

@@ -1,3 +1,7 @@
+[run]
+# Node-shebang tools (ng, eslint, prettier) run on Bun even when Node is on PATH.
+bun = true
+
 [test]
 preload = ["./src/test-setup.ts"]
 root = "src"