Add undo/redo to planner

Yjs UndoManager tracks waypoints, no-go areas, and notes. Only
local mutations (origin "local") are undoable — remote changes
from other users are not.

- Keyboard shortcuts: Ctrl/Cmd+Z (undo), Ctrl/Cmd+Shift+Z/Y (redo)
- Shortcuts suppressed in text inputs (browser-native undo there)
- Undo/redo buttons in header with disabled state
- stopCapturing on drag to isolate drag as one undo step
- All mutation sites wrapped with "local" origin
- 5 unit tests for UndoManager behavior
- Archived undo-redo change

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

packages/i18n/src/locales/de.ts

@@ -22,6 +22,8 @@ export default {
     exportRouteDesc: "GPX-Track für jede App",
     exportPlan: "Plan exportieren",
     exportPlanDesc: "Mit Wegpunkten und Sperrzonen",
+    "undo.tooltip": "Rückgängig (Strg+Z)",
+    "redo.tooltip": "Wiederholen (Strg+Umschalt+Z)",
     importGpx: "GPX importieren",
     importGpxError: "GPX-Datei konnte nicht gelesen werden. Bitte Dateiformat prüfen.",
     dropGpxHere: "GPX-Datei hier ablegen",

packages/i18n/src/locales/en.ts

@@ -22,6 +22,8 @@ export default {
     exportRouteDesc: "Clean GPX track for any app",
     exportPlan: "Export Plan",
     exportPlanDesc: "Includes waypoints and no-go areas",
+    "undo.tooltip": "Undo (Ctrl+Z)",
+    "redo.tooltip": "Redo (Ctrl+Shift+Z)",
     importGpx: "Import GPX",
     importGpxError: "Could not read GPX file. Please check the file format.",
     dropGpxHere: "Drop GPX file here",