Add multiplayer awareness: participant list, name editing, cursor styling, join/leave toasts

Implement the planner-multiplayer-awareness OpenSpec change:

- Add setUserName to use-yjs.ts that persists to localStorage and syncs via awareness
- Create ParticipantList component showing all session participants with color dots,
  host badge, and inline name editing for the local user
- Replace plain text cursor labels with SVG pointer arrows and styled name tags
  (colored background, drop shadow, rounded corners, z-index below map controls)
- Track awareness add/remove events and display auto-dismissing toasts (3s)
  when participants join or leave the session
- Add i18n keys for participant UI in English and German

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

packages/i18n/src/locales/de.ts

@@ -35,6 +35,13 @@ export default {
       shortest: "Kürzeste",
       car: "Auto",
     },
+    participants: {
+      you: "(du)",
+      host: "Host",
+      editName: "Klicke, um deinen Namen zu ändern",
+      joined: "{{name}} ist beigetreten",
+      left: "{{name}} hat die Sitzung verlassen",
+    },
     elevation: {
       gain: "Höhenmeter aufwärts",
       loss: "Höhenmeter abwärts",

packages/i18n/src/locales/en.ts

@@ -35,6 +35,13 @@ export default {
       shortest: "Shortest",
       car: "Car",
     },
+    participants: {
+      you: "(you)",
+      host: "Host",
+      editName: "Click to edit your name",
+      joined: "{{name}} joined",
+      left: "{{name}} left",
+    },
     elevation: {
       gain: "Elevation Gain",
       loss: "Elevation Loss",