Add GPX file import to planner + archive change

Two import entry points:
- Home page: "Import GPX" button next to "Start Planning"
- In-session: drag-and-drop GPX onto the map (with confirmation)

Parses GPX client-side, extracts waypoints (Douglas-Peucker) and
no-go areas from extensions. Non-GPX files show error toast.

Also:
- Fix spec drift: non-GPX drop now shows error toast (was silent)
- Add E2E tests for import button, invalid GPX, and session creation
- Archive gpx-import-planner change, sync specs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/i18n/src/locales/de.ts

@@ -22,6 +22,10 @@ export default {
     exportRouteDesc: "GPX-Track für jede App",
     exportPlan: "Plan exportieren",
     exportPlanDesc: "Mit Wegpunkten und Sperrzonen",
+    importGpx: "GPX importieren",
+    importGpxError: "GPX-Datei konnte nicht gelesen werden. Bitte Dateiformat prüfen.",
+    dropGpxHere: "GPX-Datei hier ablegen",
+    replaceRouteConfirm: "Aktuelle Route durch importierte GPX ersetzen?",
     profile: "Profil",
     connecting: "Verbinde...",
     loadingMap: "Karte wird geladen...",

packages/i18n/src/locales/en.ts

@@ -22,6 +22,10 @@ export default {
     exportRouteDesc: "Clean GPX track for any app",
     exportPlan: "Export Plan",
     exportPlanDesc: "Includes waypoints and no-go areas",
+    importGpx: "Import GPX",
+    importGpxError: "Could not read GPX file. Please check the file format.",
+    dropGpxHere: "Drop GPX file here",
+    replaceRouteConfirm: "Replace current route with imported GPX?",
     profile: "Profile",
     connecting: "Connecting...",
     loadingMap: "Loading map...",