Move Add Feed button from hamburger menu to admin area

The Add RSS modal is now opened from a button directly above the feed
list in AdminFeeds.vue instead of the hamburger menu, so feed creation
lives alongside the rest of feed management. AppNav.vue no longer
renders it.

AddUrl.vue emits an `added` event on a successful save; AdminFeeds.vue
listens for it and reloads its feed list, so a newly added feed shows
up immediately without leaving the admin page.

Also fixes a bug this move surfaced: the modal's "Close" button had no
`type`, so inside the form it defaulted to type="submit" — clicking
Close on a filled-in form silently created the feed.

vue/src/components/__tests__/RssFeeds.spec.js

@@ -37,11 +37,10 @@ describe('RssFeeds', () => {
 
     // useFeeds() returns module-level singleton refs shared across the whole
     // app (and this spec file) — reset them so state doesn't leak between tests.
-    const { feeds, showMessage, message, showModal, viewMode, currentIndex, layout } = useFeeds()
+    const { feeds, showMessage, message, viewMode, currentIndex, layout } = useFeeds()
     feeds.value = []
     showMessage.value = false
     message.value = ''
-    showModal.value = false
     viewMode.value = 'list'
     currentIndex.value = 0
     layout.value = 'list'