Show empty-state CTA on Route detail page when no waypoints

Observed: both non-maintainer users (pistazie, nelli) created a route,
never planned in it, and stopped. The Route detail page rendered as a
dead end — name, a row of buttons, then blank.

Changes:
- Detect "empty" as no geometry AND no distance (the state every new
  route has before any planning happens).
- When empty, hide the top-row button cluster (Edit in Planner / Edit
  / Export GPX) and render a centered dashed-border card in its place.
- Card shows: large headline, short body (owner-specific vs
  viewer-only copy), primary "Open in Planner" button (same action as
  the existing top-row button, just visually prominent), and a
  secondary "or upload a GPX file" link to the edit page.
- Bilingual copy added under routes.empty.*.

No non-empty route loses any UI — those paths stay identical.

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

packages/i18n/src/locales/de.ts

@@ -179,6 +179,13 @@ export default {
       noRoutesYet: "Noch keine Routen. Erstelle deine erste Route!",
       noMapPreview: "Keine Kartenvorschau",
       saveChanges: "Änderungen speichern",
+      empty: {
+        heading: "Diese Route ist leer",
+        bodyOwner: "Du hast noch keine Wegpunkte geplant. Öffne sie im Planer, um eine Strecke zu skizzieren, oder lade eine GPX-Datei hoch.",
+        bodyViewer: "Diese Route enthält noch keine Wegpunkte.",
+        openInPlanner: "Im Planer öffnen",
+        uploadGpx: "oder eine GPX-Datei hochladen",
+      },
     },
     activities: {
       title: "Aktivitäten",

packages/i18n/src/locales/en.ts

@@ -179,6 +179,13 @@ export default {
       noRoutesYet: "No routes yet. Create your first route!",
       noMapPreview: "No map preview",
       saveChanges: "Save Changes",
+      empty: {
+        heading: "This route is empty",
+        bodyOwner: "You haven't planned any waypoints yet. Open it in the Planner to sketch a path, or upload a GPX file.",
+        bodyViewer: "This route doesn't have any waypoints yet.",
+        openInPlanner: "Open in Planner",
+        uploadGpx: "or upload a GPX file",
+      },
     },
     activities: {
       title: "Activities",