feat(nav): consolidate Posts/Artists/Tags into a Browse hub

Posts, Artists, and Tags are the three 'browse the library by an axis'
surfaces; Subscriptions stays purely management (operator-asked 2026-06-09).
New BrowseView renders them as tabs (?tab=posts|artists|tags); only the active
tab mounts. The old standalone paths become redirects into the matching tab,
preserving deep-link query (/posts?post_id=N → /browse?tab=posts&post_id=N) and
keeping the route names so existing { name: 'posts'|'artists'|'tags' } links and
path pushes still resolve. Nav now reads Showcase · Gallery · Browse · Series ·
Subscriptions, with Settings pinned right.

Test: /browse resolves; /tags and /artists redirect into their tabs; a posts
deep link survives the redirect.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/test/router.spec.js

@@ -17,13 +17,29 @@ describe('router', () => {
     expect(router.resolve('/gallery').name).toBe('gallery')
   })
 
-  it('showcase, tags, artist resolve to named routes', () => {
+  it('showcase, artist resolve to named routes', () => {
     expect(router.resolve('/showcase').name).toBe('showcase')
-    expect(router.resolve('/tags').name).toBe('tags')
     expect(router.resolve('/artist/some-slug').name).toBe('artist')
     expect(router.resolve('/artist/some-slug').params.slug).toBe('some-slug')
   })
 
+  it('browse hub is reachable and old axis paths redirect into its tabs', async () => {
+    expect(router.resolve('/browse').name).toBe('browse')
+
+    await router.push('/tags')
+    expect(router.currentRoute.value.name).toBe('browse')
+    expect(router.currentRoute.value.query.tab).toBe('tags')
+
+    await router.push('/artists')
+    expect(router.currentRoute.value.query.tab).toBe('artists')
+
+    // A posts deep link survives the redirect into the hub tab.
+    await router.push({ path: '/posts', query: { post_id: '7' } })
+    expect(router.currentRoute.value.name).toBe('browse')
+    expect(router.currentRoute.value.query.tab).toBe('posts')
+    expect(router.currentRoute.value.query.post_id).toBe('7')
+  })
+
   it('series-read is an immersive route', () => {
     const r = router.resolve('/series/5/read')
     expect(r.name).toBe('series-read')