UX: remember sub/dub + filler toggle, hero time-left, next-ep CTA,

history filter chips, per-ep progress bars, stats in user menu

- sub/dub choice persists per anime (localStorage aniweb_lang_<slug>),
  so search/reload no longer drops dub watchers back to sub
- 'hide fillers' toggle persists (aniweb_ff)
- continue-watching hero shows '~Xm left' when position+duration known
- detail page: prominent 'next episode' button when the resumed ep is
  finished (auto-removed on play)
- ?history: all / watching / completed / on-hold filter chips (client-side)
- ep list: thin progress bar on partially-watched episodes
- stats page link moved into the user menu (always reachable)
- search results show a count line

static/index.html

@@ -18,6 +18,7 @@
   <div id="menubox" style="display:none">
     <button id="menu" class="tiny" aria-label="menu" aria-haspopup="true">☰</button>
     <div id="menu-items">
+      <a id="stats" class="tiny" href="?stats">stats</a>
       <button id="logout" class="tiny">logout</button>
       <button id="delacct" class="tiny danger">delete account</button>
     </div>