Optimize: drop Bootstrap CDN, add caching headers & image hints

Performance / self-hosting
- Remove the render-blocking Bootstrap 4 CDN stylesheet (~160 KB + extra
  DNS/TLS to a third party) and replace it with ~60 lines of self-hosted
  grid/utility CSS covering the small subset actually used (container, row,
  col-lg-5/6/7, order-lg, align-items-center, text-center, table-responsive,
  spacing, and the button base). Verified across desktop/tablet/mobile.
- Add a Cloudflare Pages `_headers` file: 30-day cache for static media and
  icons, 1-day for css/js, plus baseline security headers
  (X-Content-Type-Options, Referrer-Policy).
- Add decoding="async" to all lazily-loaded images.

The site now loads no third-party CSS; only Google Fonts remain external.
SCSS source kept in sync (new components/_grid partial; button base + reset
mirrored in main.scss).

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

css/components/_grid.scss

@@ -0,0 +1,49 @@
+// Minimal grid & utilities — self-hosted replacement for the small subset of
+// Bootstrap 4 this site used. Removes a render-blocking ~160 KB external
+// stylesheet. Breakpoint lg = 992px (one past $m-break: 991px).
+
+.container {
+  width: 100%;
+  max-width: 1140px;
+  margin-left: auto;
+  margin-right: auto;
+  padding-left: 15px;
+  padding-right: 15px;
+}
+
+.row {
+  display: flex;
+  flex-wrap: wrap;
+  margin-left: -15px;
+  margin-right: -15px;
+
+  > [class*="col-"] {
+    position: relative;
+    width: 100%;
+    padding-left: 15px;
+    padding-right: 15px;
+  }
+}
+
+// mobile-first: columns are full width until the lg breakpoint
+[class*="col-lg-"] { flex: 0 0 100%; max-width: 100%; }
+
+.align-items-center { align-items: center; }
+.text-center { text-align: center; }
+.mb-4 { margin-bottom: 1.5rem; }
+
+.table-responsive {
+  display: block;
+  width: 100%;
+  overflow-x: auto;
+  -webkit-overflow-scrolling: touch;
+}
+
+@media (min-width: 992px) {
+  .col-lg-5 { flex: 0 0 41.6667%; max-width: 41.6667%; }
+  .col-lg-6 { flex: 0 0 50%; max-width: 50%; }
+  .col-lg-7 { flex: 0 0 58.3333%; max-width: 58.3333%; }
+  .order-lg-1 { order: 1; }
+  .order-lg-2 { order: 2; }
+  .mb-lg-0 { margin-bottom: 0; }
+}