bootstrap removal phase 4c: nav/navbar/card custom, drop transitions

Replaced bootstrap/nav, navbar, card with used bases only (hardcoded resolved
values); dropped bootstrap/transitions entirely (no .fade/.collapse/.collapsing
used). navbar's CSS-var block was heavy, hence the big drop. Verified front-page
card and the offcanvas nav unchanged.
v4-style.min.css 104,763 -> 87,228 bytes (-58% from baseline).

css/styles.scss

@@ -72,10 +72,6 @@ $enable-negative-margins: true;
 @import '../node_modules/bootstrap/scss/type';
 @import '../node_modules/bootstrap/scss/forms';
 @import '../node_modules/bootstrap/scss/buttons';
-@import '../node_modules/bootstrap/scss/transitions';
-@import '../node_modules/bootstrap/scss/nav';
-@import '../node_modules/bootstrap/scss/navbar';
-@import '../node_modules/bootstrap/scss/card';
 
 // ---- replacements for dropped Bootstrap partials (Phase 1 + 2) ----
 
@@ -248,6 +244,45 @@ $dw-cols: 8.333333%, 16.666667%, 25%, 33.333333%, 41.666667%, 50%, 58.333333%, 6
 .list-group-horizontal > .list-group-item:first-child:not(:last-child) { border-bottom-left-radius: 0.375rem; border-top-right-radius: 0; }
 .list-group-horizontal > .list-group-item:last-child:not(:first-child) { border-bottom-left-radius: 0; border-top-right-radius: 0.375rem; }
 
+// ---- nav / navbar / card (hand-rolled; replaces those partials) ----
+// Base only; the theme customizes navbar heavily (.navbar.fixed/.visible,
+// .navbar .container, .site-title a) and self-styles the front-page .card.
+// transitions partial dropped entirely (no .fade/.collapse used).
+.nav {
+  display: flex;
+  flex-wrap: wrap;
+  list-style: none;
+  margin-bottom: 0;
+  padding-left: 0;
+}
+.navbar {
+  position: relative;
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  justify-content: space-between;
+  padding: 0.5rem 0;
+}
+.navbar-brand {
+  padding-top: 0.3125rem;
+  padding-bottom: 0.3125rem;
+  margin-right: 1rem;
+  font-size: 1.25rem;
+  white-space: nowrap;
+  text-decoration: none;
+}
+.card { // front-page only; matches former bootstrap output (bg = body-bg #d3d3d3)
+  position: relative;
+  display: flex;
+  flex-direction: column;
+  min-width: 0;
+  word-wrap: break-word;
+  color: #232323;
+  background-color: #d3d3d3;
+  border: 1px solid rgba(0, 0, 0, 0.175);
+  border-radius: 0.375rem;
+}
+
 /* Bootstrap Icons font dropped (2026-06-19): the header's 8 nav icons are now
    inline SVG in parts/header.html, so the full icon font + its ~2000 CSS rules
    are no longer imported. */