feat: add HP 12c Platinum g-shift key labels and backspace

[shanehull]

What's in

Calculator

• g + "+" → LSTx (last X recall)
• g + "×" → x² (square)
• g + "−" → backspace — deletes last digit during entry, or starts editing the displayed value when not entering
• g + "÷" → single-level undo — engine state is snapshot before each operation, restored on undo
• f/g press no longer clears the entry buffer (decimal places preserved on display)
• Entry finalized before computing ops (x², LSTx, CFj, Nj)

Frontend

• Platinum g-shift labels: ÷ ↩, × x², − ←, + LSTx
• LSTx removed from ENTER g-shift (now on +)
• Key dimensions scaled up: 66x50px (was 62x46), fonts +2px
• g-label 13px, primary 17px

What's NOT in

• Parentheses on STO/RCL (g-shift) — these are for algebraic mode, which needs infix parsing, operator precedence, and parentheses matching. A separate major feature.
• UNDO is single-level (one step), matching the HP 12c Platinum. Multi-level undo would need an undo stack.