Personal activity dashboard for signed-in users

Previously a signed-in user landing on `/` saw the same page as an
anonymous visitor: the visitor hero, the flagship marketing cards, and
the instance-wide public feed. "Home" should mean "your stuff."

The home route now branches on session: signed-out visitors keep the
marketing + public-feed layout from #303; signed-in users get a
personal dashboard showing their own activities reverse-chronologically
(all visibilities), a welcome line linking to their profile, and a
"New Activity" CTA. The public instance feed and marketing blurbs are
suppressed for signed-in users — they'd be redundant on a personal
landing surface.

Loader picks `listActivities(user.id)` vs `listRecentPublicActivities`
based on session, so only one feed query runs per request.

Updates the `journal-landing` spec with the new session-based split and
a "Personal dashboard for signed-in users" requirement.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/i18n/src/locales/de.ts

@@ -190,6 +190,7 @@ export default {
         heading: "Aktuelle Aktivitäten",
         empty: "Noch keine öffentlichen Aktivitäten.",
       },
+      dashboardEmpty: "Noch keine Aktivitäten. Erfasse oder importiere deine erste im Aktivitäten-Tab.",
       poweredBy: "Powered by trails.cool — über das Projekt",
     },
     routes: {

packages/i18n/src/locales/en.ts

@@ -190,6 +190,7 @@ export default {
         heading: "Recent activity",
         empty: "No public activities yet.",
       },
+      dashboardEmpty: "No activities yet. Record or import your first one from the Activities tab.",
       poweredBy: "Powered by trails.cool — about the project",
     },
     routes: {