Update dependencies and fix audit findings

Backend:
- cargo update: rustls 0.23.40 -> 0.23.45 (RUSTSEC-2026-0285), plus
  semver-compatible bumps (actix-web 4.15, tokio 1.53, diesel 2.3.13, ...)
- Disable actix-web's `http2` feature: the server binds plain HTTP/1.1,
  and the feature pulls in the unmaintained h2 0.3 line (RUSTSEC-2026-0258)

Frontend:
- Update all packages within their ranges, fixing 8 npm audit findings
  (nanoid, postcss, undici, ...)
- vitest 4 -> 5
- Remove unused unreadCount in RssFeeds.vue (lint error)

vue/src/components/RssFeeds.vue

@@ -1,5 +1,5 @@
 <script setup>
-import { onMounted, onBeforeUnmount, computed, nextTick, watch } from 'vue';
+import { onMounted, onBeforeUnmount, nextTick, watch } from 'vue';
 import { useFeeds } from '@/composables/useFeeds';
 
 const {
@@ -20,8 +20,6 @@ const {
   markCurrentArticleRead,
 } = useFeeds()
 
-const unreadCount = computed(() => feeds.value.filter(f => !f.read).length)
-
 const shareLabel = navigator.share ? 'Share' : 'Copy link'
 
 function scrollToNextArticle() {