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/KnowledgeView.vue

@@ -618,7 +618,7 @@ onUnmounted(() => {
 
 /* ── Filter panel ────────────────────────────────────────── */
 .filter-panel {
-  width: 180px;
+  width: var(--sidebar-width);
   flex-shrink: 0;
   padding: 16px 12px;
   border-right: 1px solid var(--color-border, rgba(255,255,255,0.06));
@@ -933,7 +933,7 @@ onUnmounted(() => {
 .minichat {
   position: absolute;
   bottom: 0;
-  left: 180px; /* align with content area past filter panel */
+  left: var(--sidebar-width); /* align with content area past filter panel */
   right: 0;
   z-index: 20;
   display: flex;