Add note context visibility in chat and standardize UI design tokens

Improve chat context: build_context() now returns metadata about auto-found
notes, emitted as an SSE event so the frontend can display context pills
showing which notes influenced the response. Users can promote notes for
deeper context (+) or exclude irrelevant ones (x). A note picker lets users
manually attach notes. Multi-word search uses per-term AND matching, and
auto-search iterates keywords individually for broader OR-style coverage.

Standardize styling: introduce CSS design tokens (--radius-sm/md/lg/pill,
--color-success/warning/overlay, --focus-ring) and migrate all components
to use them. Fix header alignment to full-width, add active nav link state,
replace hardcoded colors with CSS variables, and normalize button padding.

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

frontend/src/views/HomeView.vue

@@ -180,7 +180,7 @@ async function newChat() {
   padding: 0.75rem 1rem;
   background: var(--color-bg-card);
   border: 1px solid var(--color-border);
-  border-radius: 8px;
+  border-radius: var(--radius-md);
   text-decoration: none;
   color: var(--color-text);
   transition: border-color 0.15s;
@@ -215,11 +215,11 @@ async function newChat() {
 }
 .btn-cta {
   display: inline-block;
-  padding: 0.4rem 1rem;
+  padding: 0.45rem 1rem;
   background: var(--color-primary);
   color: #fff;
   border: none;
-  border-radius: 4px;
+  border-radius: var(--radius-sm);
   text-decoration: none;
   font-size: 0.9rem;
   cursor: pointer;