feat(phase-c): lesson plans library — browse, create, and edit lesson plans

Adds LessonPlansPage (card grid with search/filter, create dialog) and
LessonPlanDetailPage (structured editor with objectives, activities, Bloom
taxonomy tags, per-field AI suggest via ✨ button, and auto-save).

Routes: /lesson-plans and /lesson-plans/:planId wired into AppRoutes.
Nav: Lesson Plans item added to Header menu under Timetable & Classes.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/pages/Header.tsx

@@ -31,6 +31,7 @@ import Class from '@mui/icons-material/Class';
 import Book from '@mui/icons-material/Book';
 import Enrollment from '@mui/icons-material/HowToReg';
 import Lessons from '@mui/icons-material/EventNote';
+import LessonPlans from '@mui/icons-material/LibraryBooks';
 import People from '@mui/icons-material/People';
 import SchoolSettings from '@mui/icons-material/Tune';
 import { HEADER_HEIGHT } from './Layout';
@@ -279,6 +280,15 @@ const Header: React.FC = () => {
                     secondary="Weekly lesson view"
                   />
                 </MenuItem>,
+                <MenuItem key="lesson-plans" onClick={() => handleNavigation('/lesson-plans')}>
+                  <ListItemIcon>
+                    <LessonPlans />
+                  </ListItemIcon>
+                  <ListItemText
+                    primary="Lesson Plans"
+                    secondary="Plan and manage lessons"
+                  />
+                </MenuItem>,
                 <MenuItem key="student-lessons" onClick={() => handleNavigation('/student-lessons')}>
                   <ListItemIcon>
                     <Student />