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/assets/theme.css

@@ -50,6 +50,10 @@
   --radius-lg: 18px;
   --radius-pill: 9999px;
   --focus-ring: 0 0 0 2px color-mix(in srgb, var(--color-primary) 40%, transparent);
+  /* Layout */
+  --page-max-width: 1200px;
+  --page-padding-x: 1rem;
+  --sidebar-width: 260px;
 }
 
 [data-theme="dark"] {