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/src/views/PostsView.vue

@@ -3,9 +3,14 @@
     <!-- In-context view: deep-linked to one post, with bidirectional infinite
          scroll — newer posts load above, older posts below. -->
     <template v-if="postIdFilter != null">
+      <!-- Returns to whichever surface you deep-linked FROM. This used to be a
+           hard `{ name: 'posts' }`, which redirects into Browse — fine while
+           this view only ever rendered inside Browse's tab, but it now also
+           serves the front door (#387 B1), where it would have yanked the
+           operator sideways into a different view. -->
       <v-btn
         variant="text" size="small" prepend-icon="mdi-arrow-left"
-        :to="{ name: 'posts' }" class="mb-2"
+        :to="allPostsTarget" class="mb-2"
       >All posts</v-btn>
 
       <v-alert v-if="store.error" type="error" variant="tonal" closable class="mb-3">
@@ -103,6 +108,13 @@ const hasActiveFilter = computed(() =>
   artistFilter.value != null || platformFilter.value != null || searchFilter.value != null
 )
 
+// Drop only `post_id` and stay where we are — keeps Browse's `tab=posts` (and
+// any active artist/platform scope) intact instead of resetting the surface.
+const allPostsTarget = computed(() => {
+  const { post_id: _drop, ...rest } = route.query
+  return { name: route.name, query: rest }
+})
+
 // --- normal feed (downward infinite scroll) ---
 const sentinel = ref(null)
 let observer = null