Move theme CSS overrides out of the PaperMod submodule

The shorter .first-entry hero block was edited directly in
site/themes/PaperMod. The parent repo only records a submodule's commit
SHA, so those edits were never committed, and CI checks the submodule out
fresh with submodules: recursive — the deployed site rendered with
upstream's 320px/260px values.

Relocate them to site/assets/css/extended/custom.css, which PaperMod's
head.html concatenates after its core CSS, and restore the submodule to a
clean checkout of the pinned upstream commit.

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

site/assets/css/extended/custom.css

@@ -0,0 +1,16 @@
+/* Project overrides, appended after PaperMod's core CSS by the theme's
+   head.html. Keep customisations here rather than editing files inside the
+   themes/PaperMod submodule — those never get committed, and CI checks the
+   submodule out fresh, so they would be missing from the deployed site. */
+
+/* Shorter hero block: PaperMod reserves 320px / 260px for a lead image we
+   don't use, which pushed the venue tables below the fold. */
+.first-entry {
+    min-height: 180px;
+}
+
+@media screen and (max-width: 768px) {
+    .first-entry {
+        min-height: 180px;
+    }
+}