fix(plugin): hide native main-views when our panel active; BTE in-main not full-viewport

JP reported (Chrome) after b43e649:
- Sidebar buttons render ✓
- Adwright: useless middle black panel between sidebar and right area
- BTE: entire left sidebar disappears, only close-X recovers it

ROOT CAUSES:
1. webui CSS has `main.main:not(.showing-X):not(.showing-Y)... > #mainChat
   { display:flex }` which still matches when our class is svrnty-showing-*
   (not in webui's whitelist). Chat rendered alongside our panel → the
   empty void = "middle black panel".
2. BTE overlay was `position:fixed; inset:0; z-index:9991` — covered the
   whole viewport including sidebar + topbar.

FIXES:
- Adwright + BTE CSS now `main.main.svrnty-showing-<id> > .main-view
  { display:none !important }` — explicitly hides every native main-view
  (mainChat, mainSkills, mainTasks, ...) when our panel is active.
- BTE.css: overlay no longer position:fixed. Becomes a normal flex child
  of main when svrnty-showing-bte is active. Sidebar + topbar stay visible.
- bte.js: _openOverlay now appends to <main class="main"> instead of
  document.body. Migrates existing body-mounted overlay on first open.

Karpathy 4 rules: root-caused via direct CSS inspection (not guessing),
two-line CSS change per panel, no new abstractions.

static/bte.js

@@ -76,11 +76,18 @@
   }
 
   // ── Overlay lifecycle ────────────────────────────────────────────────────
+  // Mount inside <main> so the left sidebar stays visible. CSS
+  // (.svrnty-bte-overlay rules in bte.css) handles show/hide via
+  // main.svrnty-showing-bte class set by svrnty_nav.js wrapper.
   function _openOverlay() {
     let overlay = document.getElementById("svrntyBteOverlay");
+    const mountTarget = document.querySelector("main.main") || document.body;
     if (!overlay) {
       overlay = _buildOverlay();
-      document.body.appendChild(overlay);
+      mountTarget.appendChild(overlay);
+    } else if (overlay.parentElement !== mountTarget && mountTarget.tagName === "MAIN") {
+      // Migrate from body to main (handles older sessions that mounted on body).
+      mountTarget.appendChild(overlay);
     }
     overlay.hidden = false;
     _refreshGrid();