feat: add Text node type to canvas

New canvas object Text with edit/create modal: text content (single
or multi-line), font (Inter, JetBrains Mono, Serif, System Sans),
size, color, border (style + width + color), background. Double-click
on node opens edit modal. Sidebar gets "Add Text" entry below "Add Zone".

Refs #124

frontend/src/stores/canvasStore.ts

@@ -49,6 +49,8 @@ interface CanvasState {
   setNodeZIndex: (id: string, zIndex: number) => void
   editingGroupRectId: string | null
   setEditingGroupRectId: (id: string | null) => void
+  editingTextId: string | null
+  setEditingTextId: (id: string | null) => void
   createGroup: (nodeIds: string[], name: string) => void
   ungroup: (groupId: string) => void
   markSaved: () => void
@@ -71,6 +73,7 @@ export const useCanvasStore = create<CanvasState>((set) => ({
   selectedNodeId: null,
   selectedNodeIds: [],
   editingGroupRectId: null,
+  editingTextId: null,
   hideIp: false,
   scanEventTs: 0,
   fitViewPending: false,
@@ -350,6 +353,8 @@ export const useCanvasStore = create<CanvasState>((set) => ({
 
   setEditingGroupRectId: (id) => set({ editingGroupRectId: id }),
 
+  setEditingTextId: (id) => set({ editingTextId: id }),
+
   createGroup: (nodeIds, name) =>
     set((state) => {
       const PADDING_H = 24