fix(subtitle): stabilize frequency highlighting with yomitan ranks

docs/public/config.example.jsonc

@@ -136,6 +136,7 @@
       "sourcePath": "", // Optional absolute path to a frequency dictionary directory. If empty, SubMiner searches installed/default frequency-dictionary locations.
       "topX": 1000, // Only color tokens with frequency rank <= topX (default: 1000).
       "mode": "single", // single: use one color for all matching tokens. banded: use color ramp by frequency band. Values: single | banded
+      "matchMode": "headword", // Frequency lookup text selection mode. Values: headword | surface
       "singleColor": "#f5a97f", // Color used when frequencyDictionary.mode is `single`.
       "bandedColors": [
         "#ed8796",