feat(admin): link version numbers in sidebar to GitHub release notes (#566)

Closes #566.

The admin sidebar showed the running frontend + backend versions as
plain text. Wraps each version (and the "update available" indicator)
in an anchor pointing at the corresponding GitHub release tag, opening
in a new tab so the admin session isn't disrupted.

A small githubReleaseUrl helper (extracted to its own module for
testability) does the version → URL mapping. Because release-please
tags every release as `vX.Y.Z[-beta.N]`, the version string already
carries the channel suffix and a pure template covers both stable and
beta without branching.

Three unit tests pin the URL template — stable, beta-with-suffix, and
a defensive check that the leading `v` isn't double-prefixed if a
caller accidentally passes a tag-shaped value.

frontend/src/utils/githubReleaseUrl.ts

@@ -0,0 +1,14 @@
+/**
+ * Build the GitHub release page URL for a given version string.
+ *
+ * Version strings already carry their channel suffix (e.g. `3.55.0`
+ * for stable, `3.55.0-beta.0` for beta). release-please tags every
+ * release as `vX.Y.Z[-beta.N]`, so a pure string template covers
+ * both channels without branching on the suffix.
+ *
+ * Used by the admin sidebar version display to deep-link to release
+ * notes for the running version (#566) and by the update-available
+ * indicator to link to the upgrade target's notes.
+ */
+export const githubReleaseUrl = (version: string): string =>
+  `https://github.com/the-luap/picpeak/releases/tag/v${version}`;