Fix project/milestone association and redesign AppHeader navigation

Backend:
- routes/tasks.py: POST + PUT were silently dropping project_id,
  milestone_id, parent_id from request body — root cause of association
  not saving from the task editor
- routes/tasks.py: GET /api/tasks/:id now includes parent_title in
  response (secondary lookup when parent_id is set)
- routes/notes.py: add PATCH /api/notes/:id for partial updates (used
  by sub-task status toggle; PUT already existed but PATCH was missing)
- routes/projects.py: GET /api/projects/:id/notes now fetches milestone
  IDs and passes them via milestone_ids so tasks assigned to a milestone
  (but lacking project_id) are included in the project view
- services/notes.py: create_note() auto-sets project_id from milestone
  when milestone_id is provided and project_id is omitted; list_notes()
  gains milestone_ids param — when combined with project_id uses OR
  condition (project_id=X OR milestone_id IN (...))

Frontend:
- NoteEditorView: add MilestoneSelector; milestone resets when project
  changes; all save paths (save/create/auto-save) include milestone_id
- stores/notes.ts: add milestone_id to createNote + updateNote types
- TaskEditorView: sub-tasks now inherit milestone_id from parent task
- AppHeader: three-zone layout — brand left, Notes/Projects/Tasks/Chat
  centered (absolute positioning), right rail with status/theme/? and
  gear dropdown containing Settings/Users/Logs; mobile dropdown unchanged

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/stores/notes.ts

@@ -79,6 +79,7 @@ export const useNotesStore = defineStore("notes", () => {
     body: string;
     tags?: string[];
     project_id?: number | null;
+    milestone_id?: number | null;
   }): Promise<Note> {
     try {
       return await apiPost<Note>("/api/notes", data);
@@ -90,7 +91,7 @@ export const useNotesStore = defineStore("notes", () => {
 
   async function updateNote(
     id: number,
-    data: Partial<Pick<Note, "title" | "body" | "tags" | "project_id">>
+    data: Partial<Pick<Note, "title" | "body" | "tags" | "project_id" | "milestone_id">>
   ): Promise<Note> {
     try {
       const note = await apiPut<Note>(`/api/notes/${id}`, data);