fix: custom tooltip (no wiggle required), add ⓘ indicator for described units

- Replace native title-based tooltip with custom positioned div
  Native title only renders on mousemove; custom div fires on mouseenter
  after 500ms delay as intended
- Tooltip styled dark with fade-in; Dan Mode gets yellow-on-black with magenta glow
- Units with descriptions show a small superscript ⓘ on the result label

src/app.css

@@ -448,3 +448,30 @@
   line-height: 1;
   border-radius: var(--pico-border-radius);
 }
+
+/* ── Custom tooltip ────────────────────────────────────────────────────────── */
+.hu-tooltip {
+  position: fixed;
+  z-index: 9999;
+  max-width: 280px;
+  padding: 0.4rem 0.6rem;
+  border-radius: 4px;
+  font-size: 0.8rem;
+  line-height: 1.4;
+  background: #1a1a2e;
+  color: #f0f0f0;
+  box-shadow: 0 2px 8px rgba(0,0,0,0.35);
+  pointer-events: none;
+  opacity: 0;
+  transition: opacity 0.15s ease;
+  white-space: normal;
+}
+.hu-tooltip--visible {
+  opacity: 1;
+}
+[data-theme=dan] .hu-tooltip {
+  background: #000;
+  color: #ffff00;
+  border: 1px solid #ff00ff;
+  box-shadow: 0 0 8px #ff00ff;
+}