Rearrange savings lectures by splitting into two sections (#678)

This commit reorganizes the table of contents to better structure the
consumption and savings material:
- Renamed "Consumption, Savings and Capital" to "Household Problems"
- Moved optimal growth lectures (Cass-Koopmans, fiscal, AK models) to a
  new "Optimal Growth" section
- Kept household-focused lectures (cake eating, optimal growth, inventory,
  permanent income) in the Household Problems section

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

Author: jstac

lectures/_toc.yml

@@ -72,14 +72,9 @@ parts:
   - file: jv
   - file: mccall_q
   - file: odu
-- caption: Consumption, Savings and Capital
+- caption: Household Problems
   numbered: true
   chapters:
-  - file: cass_koopmans_1
-  - file: cass_koopmans_2
-  - file: cass_fiscal
-  - file: cass_fiscal_2
-  - file: ak2
   - file: cake_eating_problem
   - file: cake_eating_numerical
   - file: optgrowth
@@ -97,6 +92,14 @@ parts:
   - file: perm_income
   - file: perm_income_cons
   - file: lq_inventories
+- caption: Optimal Growth
+  numbered: true
+  chapters:
+  - file: cass_koopmans_1
+  - file: cass_koopmans_2
+  - file: cass_fiscal
+  - file: cass_fiscal_2
+  - file: ak2
 - caption: Multiple Agent Models
   numbered: true
   chapters:
@@ -131,4 +134,4 @@ parts:
   chapters:
   - file: troubleshooting
   - file: zreferences
-  - file: status
+  - file: status