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/__tests__/githubReleaseUrl.test.ts

@@ -0,0 +1,30 @@
+/**
+ * Regression coverage for the version → release-page URL template (#566).
+ *
+ * release-please tags every release as `vX.Y.Z[-beta.N]`. If anyone
+ * later refactors the helper (e.g. switches the prefix, the repo path,
+ * or strips the channel suffix), these assertions will fail loud
+ * instead of silently producing dead links in the admin sidebar.
+ */
+import { describe, expect, it } from 'vitest';
+import { githubReleaseUrl } from '../githubReleaseUrl';
+
+describe('githubReleaseUrl', () => {
+  it('maps a stable version to its tag page', () => {
+    expect(githubReleaseUrl('3.55.0')).toBe(
+      'https://github.com/the-luap/picpeak/releases/tag/v3.55.0',
+    );
+  });
+
+  it('maps a beta version (with channel suffix) to its tag page', () => {
+    expect(githubReleaseUrl('3.55.0-beta.0')).toBe(
+      'https://github.com/the-luap/picpeak/releases/tag/v3.55.0-beta.0',
+    );
+  });
+
+  it('does not double-prefix the v when given a bare version', () => {
+    // Defensive — the helper is the single source of truth for the
+    // leading "v". Callers must pass the bare version string.
+    expect(githubReleaseUrl('3.55.0')).not.toContain('vv');
+  });
+});

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}`;