Fix the accessibility findings from the audit

Contrast: --mute-2 and --trash both failed AA in both themes — every
10px mono label (recent chapter, section headings, chapter hint) sat
between 2.5:1 and 3.7:1, and the trash icon, the one irreversible
control, was the least visible thing in the strip. Retuned per theme:
labels now 4.8-5.2:1, trash 4.3-4.5:1 against a 3:1 requirement.

Focus: the search input clears its own outline and the underline lives
on the parent, so :focus-within on .searchbar carries the ring.

The inline error slot gets role=status, so a failed write is announced
rather than looking like an ignored tap.

The tab strip drops role=tablist/tab: these are real links that change
the URL, and the roles promised a tabpanel contract that was never
implemented, leaving aria-selected unset on every one. aria-current
carries the state instead.

prefers-reduced-motion now names pseudo-elements — `*` does not match
them, so the busy bar and the error dot, both ::before, kept animating
for the users who asked them not to.

Log out was a 54x15 target; an ::after pad makes it 78x45 without
moving the label.

Also: the two dark-tuned hex borders in the desktop block become
theme-aware tokens, and the preload swaps mono for DM Sans, the larger
face and the one the body text needs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/static/filter.js

@@ -20,8 +20,11 @@
 })();
 
 function setActiveTab(el) {
-  el.parentElement.querySelectorAll("[role=tab]").forEach(function (t) {
-    t.classList.toggle("active", t === el);
+  el.parentElement.querySelectorAll("a").forEach(function (t) {
+    var on = t === el;
+    t.classList.toggle("active", on);
+    if (on) t.setAttribute("aria-current", "page");
+    else t.removeAttribute("aria-current");
   });
 }