Add an About section to the account menu (#51)

Shows the app version, links the AGPL-3.0-or-later license text, and
links the source repo -- AGPL's own suggested-usage text recommends
exactly this ("if your software can interact with users remotely...
its interface could display a 'Source' link"), not just a courtesy
credits screen.

Version comes from package.json at build time via a Vite `define`
(__APP_VERSION__), so it can't drift from what's actually released.
License text is served at /LICENSE via a frontend/public/ symlink to
the repo-root LICENSE, the same pattern already used for the user
guide. Also bumps both package manifests to 1.0.0 ahead of tagging
the first release.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/AboutModal.css

@@ -0,0 +1,27 @@
+.about-modal-brand {
+  display: flex;
+  align-items: center;
+  gap: var(--sp-3);
+  margin-bottom: var(--sp-4);
+}
+
+.about-modal-logo {
+  width: 40px;
+  height: 40px;
+  flex: none;
+}
+
+.about-modal-name {
+  font-weight: 700;
+  font-size: 1.05rem;
+}
+
+.about-modal-version {
+  color: var(--ds-muted);
+  font-size: 0.85rem;
+}
+
+.about-modal-line {
+  margin: 0 0 var(--sp-2);
+  font-size: 0.9rem;
+}