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/ChatView.vue

@@ -1037,7 +1037,7 @@ onUnmounted(() => {
 }
 
 .chat-sidebar {
-  width: 260px;
+  width: var(--sidebar-width);
   min-width: 200px;
   display: flex;
   flex-direction: column;
@@ -1360,14 +1360,14 @@ onUnmounted(() => {
   padding: 1rem 1.5rem;
   display: flex;
   flex-direction: column;
-  max-width: 1200px;
+  max-width: var(--page-max-width);
   margin: 0 auto;
   width: 100%;
 }
 
 .context-sidebar {
-  width: 220px;
-  min-width: 180px;
+  width: var(--sidebar-width);
+  min-width: 200px;
   background: var(--color-bg-secondary);
   display: flex;
   flex-direction: column;
@@ -1578,7 +1578,7 @@ details[open] .thinking-summary::before {
 
 /* Input wrapper */
 .input-wrapper {
-  max-width: 1200px;
+  max-width: var(--page-max-width);
   margin: 0 auto 2rem;
   padding: 0 1rem;
   width: 100%;
@@ -1914,7 +1914,7 @@ details[open] .thinking-summary::before {
     z-index: 100;
     transform: translateX(-100%);
     transition: transform 0.25s ease;
-    width: 260px;
+    width: var(--sidebar-width);
   }
   .chat-sidebar.open {
     transform: translateX(0);