Misc · Numbers: a calculator and a unit converter

misc::Calculator works a typed sum in forms::evaluate's precedence,
from a pad or the keyboard; Enter is equals while the calculator
itself has focus, and Escape clears or passes on. misc::UnitConverter
reads an amount in another unit across seven kinds, with a swap.
Both show numbers through format::significant, which rounds through
Rust's exponent form.

assets/icons/arrow-down-up.svg

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+<!-- @license lucide-static v1.48.0 - ISC -->
+<svg
+  class="lucide lucide-arrow-down-up"
+  xmlns="http://www.w3.org/2000/svg"
+  width="24"
+  height="24"
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+  fill="none"
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+  stroke-linecap="round"
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+>
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+  <path d="M7 20V4" />
+  <path d="m21 8-4-4-4 4" />
+  <path d="M17 4v16" />
+</svg>

assets/icons/delete.svg

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+<!-- @license lucide-static v1.48.0 - ISC -->
+<svg
+  class="lucide lucide-delete"
+  xmlns="http://www.w3.org/2000/svg"
+  width="24"
+  height="24"
+  viewBox="0 0 24 24"
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+  stroke="currentColor"
+  stroke-width="2"
+  stroke-linecap="round"
+  stroke-linejoin="round"
+>
+  <path d="M10 5a2 2 0 0 0-1.344.519l-6.328 5.74a1 1 0 0 0 0 1.481l6.328 5.741A2 2 0 0 0 10 19h10a2 2 0 0 0 2-2V7a2 2 0 0 0-2-2z" />
+  <path d="m12 9 6 6" />
+  <path d="m18 9-6 6" />
+</svg>