Rename the darker tone to AMOLED; file icons are always colored

- Tone: the "darker" option is labelled "AMOLED" in all 7 locales — it is
  the true-black AMOLED palette. The persisted value stays "darker", so no
  settings migration.
- File icons: the Plain/Multi option is gone and the per-category glyphs +
  colors are always on. Removed the iconColors setting (type, default,
  signal, setter, options row, strings) and the plain branch in
  file-icons.ts. A stale "iconColors" key in settings.json is carried
  harmlessly and dropped on the next save.

Co-Authored-By: Claude <noreply@anthropic.com>

ui/src/styles.css

@@ -30,7 +30,7 @@
   --input: oklch(0.92 0.004 286.32);
   --ring: oklch(0.705 0.015 286.067);
   --radius: 0.5rem;
-  /* file-type icon colors ("Multi" mode) — fixed category hues, muted chroma */
+  /* file-type icon colors — fixed category hues, muted chroma */
   --icon-folder: oklch(0.58 0.13 250);
   --icon-link: oklch(0.56 0.1 190);
   --icon-image: oklch(0.58 0.13 295);