feat(ui): add consistent page-header layout and styles

Introduce a shared page-header/page-heading pattern and CSS to
standardize headers across top-level sections. Components were
updated to adopt the new classes and slightly smaller icon sizing
for a more compact, consistent header appearance. This improves
visual consistency and makes future header adjustments simpler.

- Add reusable page-header styles for spacing, borders, and bg
- Standardize icon sizing and heading layout across screens

src/app.css

@@ -9069,3 +9069,36 @@ input:focus, textarea:focus, select:focus { box-shadow: 0 0 0 3px color-mix(in s
   --color-sync-ahead: #9a5200;
   --color-sync-behind: #0755c8;
 }
+
+/* Consistent page headers for repositories, pull requests, and issues. */
+section > header.page-header.page-header {
+  box-sizing: border-box;
+  display: flex;
+  flex-shrink: 0;
+  min-height: 54px;
+  height: auto;
+  align-items: center;
+  justify-content: space-between;
+  gap: 16px;
+  padding: 8px 24px;
+  border-bottom: 1px solid var(--color-border-subtle);
+  background: color-mix(in srgb, var(--app-bg) 94%, var(--color-surface));
+}
+.page-header .page-heading {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  min-width: 0;
+}
+.page-header .page-heading > svg {
+  flex: 0 0 auto;
+  color: var(--color-accent);
+}
+.page-header.page-header .page-heading h1 {
+  margin: 0;
+  color: var(--color-ink);
+  font-size: 14px;
+  font-weight: 700;
+  line-height: 20px;
+  letter-spacing: normal;
+}