fix: uncontrolled input — stop re-setting value prop on every keystroke, killing focus

src/components/UnitInput.svelte

@@ -1,7 +1,16 @@
 <script lang="ts">
   /**
-   * UnitInput — controlled numeric input.
-   * Phase 2: number input only. NL field added in Phase 3.
+   * UnitInput — uncontrolled numeric input with one-way push to parent.
+   *
+   * The input owns its own DOM value. We only push changes upward via onchange.
+   * We deliberately do NOT set value={value} on the input element — that would
+   * cause Svelte to re-render the input on every parent state update, resetting
+   * the cursor position and stealing focus mid-typing.
+   *
+   * The parent can pass a new `value` prop if it needs to imperatively set the
+   * input (e.g. Ctrl+click adopts a converted value). We detect that via $effect
+   * and update the DOM node directly, only when the value actually differs from
+   * what's currently in the input.
    */
 
   let {
@@ -12,6 +21,18 @@
     onchange: (v: number) => void;
   } = $props();
 
+  let inputEl = $state<HTMLInputElement | null>(null);
+
+  // Sync parent-driven value changes into the DOM — but only when the parent
+  // is actually setting a new value (e.g. Ctrl+click), not on every keystroke.
+  $effect(() => {
+    if (!inputEl) return;
+    const current = parseFloat(inputEl.value);
+    if (isNaN(current) || current !== value) {
+      inputEl.value = String(value);
+    }
+  });
+
   function handleInput(e: Event) {
     const input = e.currentTarget as HTMLInputElement;
     const parsed = parseFloat(input.value);
@@ -22,9 +43,9 @@
 </script>
 
 <input
+  bind:this={inputEl}
   type="number"
   step="any"
-  value={value}
   oninput={handleInput}
   aria-label="Input value"
 />

src/lib/types.ts

@@ -24,6 +24,9 @@ export interface Unit {
 
   /** Group this unit belongs to, or null if orphaned. */
   group: string | null;
+
+  /** Optional human-readable description shown as a tooltip in the converter. */
+  description?: string;
 }
 
 /** A group of units sharing a base unit and a universal scale factor. */