docs: renewed README and light/dark diagonal screenshots; wider hashtag tiles

- README: current features (journal, focus mode, push, themes, sharing
  into the inbox), outgoing connections, :latest instead of :stable,
  VAPID_* and LEGAL_CUSTOM_LINK_*, new journal and phone screenshots
- e2e/screenshots.sh: light and dark in the same state joined along a
  diagonal, own container/port per run
- hashtag tiles at least 18rem wide

chore(release): v0.15.0

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/features/workspace/WorkspaceView.tsx

@@ -2102,7 +2102,8 @@ export const WorkspaceView: React.FC<WorkspaceViewProps> = ({
                   )}
                 </div>
               ) : (
-                <div className="grid grid-cols-1 sm:grid-cols-2 md:grid-cols-3 xl:grid-cols-4 2xl:grid-cols-5 gap-3.5">
+                <div className="grid grid-cols-[repeat(auto-fill,minmax(min(100%,18rem),1fr))] gap-3.5">
+                  {/* As many columns as fit with at least 18rem each, so tag names and note titles have room */}
                   {filteredTags.map((t) => {
                     const meta = tagsMetadataMap.get(t.tag.toLowerCase());
                     return (