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/types/chat.ts

@@ -25,6 +25,12 @@ export interface OllamaModel {
   size: number;
 }
 
+export interface ContextMeta {
+  context_note_id: number | null;
+  context_note_title: string | null;
+  auto_notes: { id: number; title: string }[];
+}
+
 export interface OllamaStatus {
   ollama: "available" | "unavailable";
   model: "ready" | "not_found";