Polish: plain unfound word-search chips + no signed-out flash on refresh

- Word Search: unfound words were tinted (accent-soft background) like found
  ones, so the remaining words were hard to spot as the board filled. Unfound
  chips are now plain (transparent + a light outline); found words keep their
  grid colour. Easy to see what's left.
- Auth: a refresh briefly flashed the signed-out header until /api/auth/me
  returned. Now the last-known user is cached and hydrated immediately, so the
  signed-in UI paints at once; the session is still revalidated every load (a
  stale/expired one corrects within a beat) and the cache is cleared on logout.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/components/WordSearchGame.svelte

@@ -221,8 +221,12 @@
   .plabel { text-transform: uppercase; letter-spacing: 0.12em; font-size: 0.62rem; font-family: var(--label);
             color: var(--muted); text-align: center; margin: 0 0 10px; }
   .words { list-style: none; display: flex; flex-wrap: wrap; gap: 8px; justify-content: center; padding: 0; margin: 0; }
+  /* Unfound words are plain (transparent + outline) so it's easy to see what's
+     left as the board fills; found words get their grid colour via inline style. */
   .words li { font-family: var(--label); font-size: 0.82rem; letter-spacing: 0.04em; color: var(--ink);
-              padding: 4px 11px; border-radius: 999px; background: var(--accent-soft); transition: background 0.2s ease; }
+              padding: 4px 11px; border-radius: 999px; background: transparent;
+              border: 1px solid var(--line); transition: background 0.2s ease, border-color 0.2s ease; }
+  .words li.got { border-color: transparent; }
   .hint { text-align: center; color: var(--muted); font-size: 0.84rem; margin: 0; }
   .result { text-align: center; }
   .rmark { font-family: var(--serif); font-style: italic; color: var(--accent-deep); font-size: 1.2rem; margin: 0 0 12px; }