feat: add gzip toggle and cache mode (normal / no-cache / bust)

- Cache-mode selector: Normal, No-cache headers, Cache-bust URL+headers
- Cache-bust appends random ?_cb= per iteration to bypass CDN/proxy
- gzip toggle: on = k6 default (Accept-Encoding: gzip/br), off = identity
- Both options stored in DB with non-destructive migration
- History items show cache-mode and gzip pills
- Schema migration handles existing DBs gracefully

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/style.css

@@ -112,6 +112,46 @@ textarea { font-family: var(--mono); resize: vertical; }
   cursor: pointer;
   transition: background .15s;
 }
+
+/* TOGGLE SWITCH */
+.toggle-row { display: flex; align-items: center; gap: .75rem; margin-top: .15rem; }
+.toggle { display: flex; align-items: center; gap: .55rem; cursor: pointer; }
+.toggle input { display: none; }
+.toggle-track {
+  width: 38px; height: 20px;
+  background: var(--border);
+  border-radius: 999px;
+  position: relative;
+  transition: background .2s;
+  flex-shrink: 0;
+}
+.toggle-track::after {
+  content: '';
+  position: absolute;
+  width: 14px; height: 14px;
+  background: #fff;
+  border-radius: 50%;
+  top: 3px; left: 3px;
+  transition: transform .2s;
+}
+.toggle input:checked + .toggle-track { background: var(--accent); }
+.toggle input:checked + .toggle-track::after { transform: translateX(18px); }
+.toggle-label { font-size: .88rem; color: var(--text); }
+
+/* PILLS */
+.pill {
+  display: inline-block;
+  font-size: .72rem;
+  font-weight: 600;
+  padding: .15rem .5rem;
+  border-radius: 999px;
+  letter-spacing: .04em;
+}
+.pill.cached   { background: rgba(34,197,94,.12);  color: #4ade80; }
+.pill.no-cache { background: rgba(245,158,11,.12); color: #fbbf24; }
+.pill.bust     { background: rgba(239,68,68,.12);  color: #f87171; }
+.pill.gzip     { background: rgba(124,58,237,.15); color: var(--accent2); }
+.pill.no-gzip  { background: rgba(100,116,139,.12); color: var(--muted); }
 .btn-secondary:hover { background: rgba(124,58,237,.15); }
 
 /* RESULT */