refactor: DRY layout bounds via --page-max-width and --sidebar-width CSS vars

Add --page-max-width (1200px), --page-padding-x (1rem), and --sidebar-width (260px)
to theme.css so all views share a single source of truth.

- HomeView: 1100px → var(--page-max-width) (aligns with all other views)
- NotesListView, TasksListView, ProjectListView, ProjectView, CalendarView: var(--page-max-width)
- ChatView: sidebar + context sidebar → var(--sidebar-width); inner message/input
  column max-widths → var(--page-max-width)
- KnowledgeView: filter panel 180px → var(--sidebar-width); minichat left offset updated

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

frontend/src/views/NotesListView.vue

@@ -192,9 +192,9 @@ function onOffsetUpdate(offset: number) {
 
 <style scoped>
 .notes-list {
-  max-width: 1200px;
+  max-width: var(--page-max-width);
   margin: 2rem auto;
-  padding: 0 1rem;
+  padding: 0 var(--page-padding-x);
   overflow-x: clip;
 }
 .header {