feat: the front door answers "what arrived?" (milestone 387 step B1)

Moves FRONT_DOOR from /showcase to /latest. Showcase is a random
TABLESAMPLE — lean-back, and it can never tell you anything is wrong.
The feed is the only view where a failing source surfaces on its own,
as a creator who has gone quiet. The per-artist "new since last visit"
badges (#597) were a workaround for this view not being the door.

Showcase is demoted to a nav entry, not removed. Nothing is being
replaced, so rule 22's delete-the-legacy-path does not apply.

Deviation from the filed plan, deliberate: the step said write a new
LatestView.vue. Rejected — PostsView is ALREADY a self-contained feed
(own container, own store, infinite scroll, filters, deep-link
anchoring, empty state), and Browse only ever wrapped it in a tab
strip. A new view would have duplicated 231 working lines to gain
nothing. Mounting PostsView directly at its own route IS the whole
difference the promotion was after: a door you arrive at, not a hub
you navigate out of. Rule 28.

Backend untouched, as scoped — PostFeedService.scroll already does
cursor-paginated newest-first.

Two things this shook loose:

PostsView's deep-link "All posts" button was a hard `{ name: 'posts' }`,
which redirects into Browse. Correct while the view only ever rendered
inside Browse's tab; from the front door it would have yanked the
operator sideways into a different surface. Now returns to the current
route minus post_id, so Browse keeps its tab and any active scope.

The README claimed "A Showcase front page". That block is the SOURCE
the release notes quote (scripts/release_notes.py product_overview),
not a generated copy, so it is fixed here — a document contradicting
the code is the characteristic defect of the public-surface area.

No stickyChrome on the route: unlike Browse/Gallery/Settings this view
has no sticky sub-header for the nav to butt against.

The router spec pinned FRONT_DOOR to /showcase and now pins /latest,
plus that Showcase stayed reachable and in the nav — the demotion is
asserted, not assumed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LNXXULQDjVZmbuNa2G9mD9

frontend/test/router.spec.js

@@ -2,8 +2,29 @@ import { describe, it, expect } from 'vitest'
 import router, { FRONT_DOOR } from '../src/router.js'
 
 describe('router', () => {
-  it('FRONT_DOOR defaults to /showcase', () => {
-    expect(FRONT_DOOR).toBe('/showcase')
+  it('FRONT_DOOR is the post feed', () => {
+    // Moved from /showcase in #387 B1. Asserted on the constant rather than on
+    // a navigation so the intent is pinned: the door is the feed, and moving it
+    // again should be a deliberate edit to a failing test, not a quiet change.
+    expect(FRONT_DOOR).toBe('/latest')
+  })
+
+  it('/latest is the feed, mounted outside the Browse tab strip', () => {
+    const r = router.resolve('/latest')
+    expect(r.name).toBe('latest')
+    // A nav entry in its own right (meta.title is what TopNav lists on), and
+    // first in the order.
+    expect(r.meta.title).toBe('Latest')
+    expect(r.meta.navOrder).toBe(5)
+    // Deliberately NOT stickyChrome — it has no sticky sub-header for the nav
+    // to butt against, unlike Browse/Gallery/Settings.
+    expect(r.meta.stickyChrome).toBeUndefined()
+  })
+
+  it('showcase is demoted, not removed — still reachable and still in the nav', () => {
+    const r = router.resolve('/showcase')
+    expect(r.name).toBe('showcase')
+    expect(r.meta.title).toBe('Showcase')
   })
 
   it('/ redirects to FRONT_DOOR', async () => {