feat(design): apply kami serif 500 to app headings

Extend the @layer base h1/h2/h3 rule so every application-level heading
inherits the kami serif stack at weight 500, matching design.md rule 4
(serif carries headline authority) and rule 5 (single serif weight is
the design signature). Prose headings inside .paperclip-markdown already
inherit the same stack via the prose rules added in the token rewrite.

Components that override font-family/font-weight on headings will
continue to win via CSS specificity; the base rule sets the kami default
for every heading that does not opt out.

Typecheck and build:verify clean.

src/renderer/styles.css

@@ -201,6 +201,9 @@
   h1,
   h2,
   h3 {
+    font-family: var(--font-serif);
+    font-weight: 500;
+    letter-spacing: -0.005em;
     text-wrap: balance;
   }
   /* Prevent double-tap-to-zoom on interactive elements for mobile */