Built out the full UI with four tabs — encode, decode, analyze, and trace.

The analyze tab was the most interesting to build. Had to implement the
SantaLucia 1998 nearest-neighbor thermodynamic model to estimate melting
temperature. It uses ΔH and ΔS values for every possible base pair
combination to predict how stable a strand is under heat. For short
strands under 14 bases it falls back to the simpler Wallace rule.

Also learned what GC content actually means — G and C form 3 hydrogen
bonds when they pair, A and T only form 2. So a strand with good GC
balance is physically stronger. The ideal range for synthesis is 40–60%.