ReaderFooter: Add a compact prefix for frontlight_warmth

Fix #8398

Also, tweak the actual codepoints being used so that they somewhat look
like their icon brothers. May or may not actually be compact :D.

frontend/apps/reader/modules/readerfooter.lua

@@ -95,7 +95,8 @@ local symbol_prefix = {
         percentage = nil,
         book_time_to_read = nil,
         chapter_time_to_read = BD.mirroredUILayout() and "«" or "»",
-        frontlight = "*",
+        frontlight = "✺",
+        frontlight_warmth = "⊛",
         -- @translators This is the footer compact item prefix for memory usage.
         mem_usage = C_("FooterCompactItemsPrefix", "M"),
         wifi_status = "",