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.tsx

@@ -0,0 +1,56 @@
+import logo from '../assets/logo.png'
+import './Modal.css'
+import './AboutModal.css'
+
+interface AboutModalProps {
+  onClose: () => void
+}
+
+// AGPL-3.0 itself recommends this: "if your software can interact with
+// users remotely through a computer network, you should also make sure
+// that it provides a way for users to get its source... its interface
+// could display a 'Source' link" -- this modal is that link, not just a
+// courtesy credits screen.
+const SOURCE_URL = 'https://git.darksingularity.org/DarkSingularity/ds-chat'
+
+export function AboutModal({ onClose }: AboutModalProps) {
+  return (
+    <div className="modal-scrim" onClick={onClose}>
+      <div className="modal about-modal" onClick={(e) => e.stopPropagation()}>
+        <div className="modal-header">
+          <h2>About</h2>
+          <button type="button" className="modal-close" onClick={onClose} aria-label="Close">
+            &times;
+          </button>
+        </div>
+
+        <div className="about-modal-brand">
+          <img src={logo} alt="" className="about-modal-logo" />
+          <div>
+            <div className="about-modal-name">DS Chat</div>
+            <div className="about-modal-version">Version {__APP_VERSION__}</div>
+          </div>
+        </div>
+
+        <p className="about-modal-line">
+          Licensed under the{' '}
+          <a href="/LICENSE" target="_blank" rel="noopener noreferrer">
+            GNU Affero General Public License v3.0 (or later)
+          </a>
+          .
+        </p>
+        <p className="about-modal-line">
+          <a href={SOURCE_URL} target="_blank" rel="noopener noreferrer">
+            Source code
+          </a>
+        </p>
+
+        <div className="modal-actions" style={{ marginTop: '1rem' }}>
+          <button type="button" className="btn-secondary" onClick={onClose}>
+            Close
+          </button>
+        </div>
+      </div>
+    </div>
+  )
+}