Enhance Template Logic: Nested If/Elif/Else, Parentheses, and Logical Operators

[arbuthnot-eth]

Users need more powerful template logic when composing notes. This feature adds a robust expression evaluator and a nesting-aware IF subsystem to support:

• Parentheses and precedence for complex conditions.
• Logical operators as words and symbols (not/and/or and !/&&/||).
• Comparison operators and JS-like truthiness.
• Elif/elseif chains and deeply nested branches.
• Conditional {% set %} assignments that only run in the selected branch.
  This substantially improves template expressiveness while keeping behavior predictable and safe.