feat: add About well to settings with build info and debug easter egg

Add a new well at the bottom of the settings view that displays:
- License (GPL-3.0)
- Author (sneak)
- Version (from package.json)
- Build date (injected at build time)
- Git commit short hash (linked to Gitea commit URL)

Build-time injection: build.js now reads the git commit hash and version
from package.json, injecting them via esbuild define constants. The
Dockerfile and Makefile pass commit hashes as build args so the info is
available even when .git is excluded from the Docker context.

Easter egg: clicking the version number 10 times reveals a hidden debug
well below the About well, containing a toggle for debug mode. The debug
mode flag is persisted in state and enables verbose console logging via
the runtime debug flag in the logger.

closes #144

src/shared/buildInfo.js

@@ -0,0 +1,35 @@
+// Build-time constants injected by esbuild define in build.js.
+// These globals are replaced at bundle time with string literals.
+
+/* global __BUILD_VERSION__, __BUILD_LICENSE__, __BUILD_AUTHOR__,
+          __BUILD_COMMIT__, __BUILD_COMMIT_FULL__, __BUILD_DATE__ */
+
+const BUILD_VERSION =
+    typeof __BUILD_VERSION__ !== "undefined" ? __BUILD_VERSION__ : "dev";
+const BUILD_LICENSE =
+    typeof __BUILD_LICENSE__ !== "undefined" ? __BUILD_LICENSE__ : "GPL-3.0";
+const BUILD_AUTHOR =
+    typeof __BUILD_AUTHOR__ !== "undefined"
+        ? __BUILD_AUTHOR__
+        : "sneak <sneak@sneak.berlin>";
+const BUILD_COMMIT =
+    typeof __BUILD_COMMIT__ !== "undefined" ? __BUILD_COMMIT__ : "unknown";
+const BUILD_COMMIT_FULL =
+    typeof __BUILD_COMMIT_FULL__ !== "undefined"
+        ? __BUILD_COMMIT_FULL__
+        : "unknown";
+const BUILD_DATE =
+    typeof __BUILD_DATE__ !== "undefined" ? __BUILD_DATE__ : "unknown";
+
+const GITEA_COMMIT_URL =
+    "https://git.eeqj.de/sneak/AutistMask/commit/" + BUILD_COMMIT_FULL;
+
+module.exports = {
+    BUILD_VERSION,
+    BUILD_LICENSE,
+    BUILD_AUTHOR,
+    BUILD_COMMIT,
+    BUILD_COMMIT_FULL,
+    BUILD_DATE,
+    GITEA_COMMIT_URL,
+};