Fix wave round 2: serif toggle actually toggles; empty number settings stay empty

- The session-name-serif toggle looked dead because --font-title was an
  @theme INLINE token: the .font-title utility carried a baked copy of the
  family, so the runtime var the toggle wrote was never read by any
  consumer. Moved --font-title to a plain @theme block — the utility now
  reads var(--font-title) and the :root factory default emits — the toggle
  flips the session name's computed font (lh-cmu ↔ Source Sans 3).
- Empty reading-font-size and heading-weight commits wrote the clamp
  minimum (Number("") is 0 → 12 / 100) instead of unsetting. Empty now
  omits the key — absence is the delta schema's unset for numbers — so the
  field stays empty, the record unsets, and the hint carries the resolved
  default (16px / 900). Typed values still clamp visibly (-5 → 12).

Gates: tsc clean, 866/866, lint 0, build ok. Live-verified on :3001.

components/settings/appearance-section.tsx

@@ -73,6 +73,13 @@ export function AppearanceSection({
 					ariaLabel="prose font size"
 					testid="input-prose-size"
 					onCommit={(v) => {
+						// Empty = factory default (owner, fix wave): omit the key so the record
+						// UNSETS and the field stays empty (absence, not null, is the
+						// unset for numbers). Number("") is 0, so the old clamp wrote 12.
+						if (v === "") {
+							onSectionChange({ ...appearance, proseFontSize: undefined });
+							return;
+						}
 						const n = Number(v);
 						if (!Number.isFinite(n)) return; // bad input: no write, factory stays
 						onSectionChange({ ...appearance, proseFontSize: Math.min(24, Math.max(12, Math.round(n))) });
@@ -94,6 +101,12 @@ export function AppearanceSection({
 					ariaLabel="heading weight"
 					testid="input-heading-weight"
 					onCommit={(v) => {
+						// Empty = factory default (owner, fix wave): Number("") is 0, so
+						// Absence is the unset representation — the old clamp wrote 100.
+						if (v === "") {
+							onSectionChange({ ...appearance, mdRendering: { ...md, headingWeight: undefined } });
+							return;
+						}
 						const n = Number(v);
 						if (!Number.isFinite(n)) return;
 						onSectionChange({