Add keyboard navigation: e/c/slash shortcuts, j/k list nav, focus visibility

- App.vue: e → edit on viewer pages; / → focus search (custom event); c → focus
  chat on home or navigate to /chat; update shortcuts panel with Lists + Chat sections
- theme.css: add a:focus-visible to focus-ring rules (router-link cards now show
  visible keyboard focus outline)
- SearchBar.vue: expose focus() via defineExpose for cross-component focus dispatch
- NotesListView / TasksListView: j/k vim-style navigation with kb-active-item
  highlight; listen for shortcut:focus-search; cleanup on unmount
- HomeView.vue: listen for shortcut:focus-chat, call chatInputRef.focus()

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

frontend/src/views/HomeView.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { ref, computed, onMounted, nextTick } from "vue";
+import { ref, computed, onMounted, onUnmounted, nextTick } from "vue";
 import { apiGet } from "@/api/client";
 import type { Note, NoteListResponse } from "@/types/note";
 import type { Task, TaskListResponse } from "@/types/task";
@@ -206,6 +206,17 @@ function onStatusToggle(id: number, status: TaskStatus) {
 // ─── Chat widget ──────────────────────────────────────────────────────────────
 
 const chatInputRef = ref<{ focus: () => void } | null>(null);
+
+function onFocusChatShortcut() {
+  chatInputRef.value?.focus();
+}
+
+onMounted(() => {
+  document.addEventListener("shortcut:focus-chat", onFocusChatShortcut);
+});
+onUnmounted(() => {
+  document.removeEventListener("shortcut:focus-chat", onFocusChatShortcut);
+});
 const chatStore = useChatStore();
 
 chatStore.fetchStatus().then(() => {