feat: silent pre-indexing with configurable status bar indicator

Port PR #979's silent pre-indexing and add an Indexing Display setting
(Popup / Status Bar Text / Status Bar Icon) so users can choose how
indexing feedback is shown.

Silent pre-indexing runs on text-only penultimate pages when a status
bar option is selected, with a standard requestUpdate to clear the
indicator. Image pages skip silent indexing to avoid e-ink grayscale
pipeline conflicts; the normal popup handles those transitions. Direct
chapter jumps always show the original small popup regardless of setting.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/I18n/I18nKeys.h

@@ -390,6 +390,10 @@ enum class StrId : uint16_t {
   STR_TZ_HAWAII,
   STR_TZ_CUSTOM,
   STR_SET_UTC_OFFSET,
+  STR_INDEXING_DISPLAY,
+  STR_INDEXING_POPUP,
+  STR_INDEXING_STATUS_TEXT,
+  STR_INDEXING_STATUS_ICON,
   // Sentinel - must be last
   _COUNT
 };