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

data/units.json

@@ -62,7 +62,9 @@
       "labelPlural": "Metric fucktons",
       "symbol": "mfcktn",
       "group": "dickloads",
-      "toBase": 170.2399999998
+      "toBase": 170.2399999998,
+      "description": "Trivia: although the name is \"metric fuckton,\" it is actually equal to two long tons!",
+      "hidden": false
     },
     {
       "id": "assload",