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

README.md

@@ -13,8 +13,9 @@ tags it, and gives you something better than a folder full of images to look
 through afterwards.
 
 - **Gallery and browsing.** Images, videos and multi-page works, organised by
-  artist, tag, post and series. A Showcase front page, a filterable gallery, a
-  similarity-driven Explore view, and a page-turning reader for series.
+  artist, tag, post and series. A newest-first feed of what just arrived as the
+  front page, a random Showcase, a filterable gallery, a similarity-driven
+  Explore view, and a page-turning reader for series.
 - **Subscriptions.** Follows creators on Patreon, SubscribeStar, Pixiv and
   anything `gallery-dl` supports, on a schedule. Handles paywalled posts using
   your own logged-in session.