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>

docs/architecture.md

@@ -234,8 +234,8 @@ component; tokens → `ui/src/styles.css`.
   (dotted lookup, `{param}` interpolation, `{one,other}` plural maps via
   `Intl.PluralRules`); missing keys fall back to English then the raw key.
   **Reactive by design**: `t()` reads the `locale` signal, so every template
-  binding that calls it re-renders live on a language switch (same call-stack
-  tracking as `iconClass`) — no pipe. Also exposes locale-aware `date()`/`size()`
+  binding that calls it re-renders live on a language switch (call-stack
+  tracking, see gotchas.md) — no pipe. Also exposes locale-aware `date()`/`size()`
   /`speed()` so the file list reformats on switch. `SettingsService.applyLocale`
   resolves the `locale` setting (`system` → `navigator.language`) into it, on
   boot and on the `settingsChanged` broadcast (so editor/manager windows follow).