v2.7.0: Time-of-day greeting, bearing indicator, and empty-sky timer

Three small usability additions:
- Splash screen now shows a "Good morning/day/evening" greeting under
  the title, based on local time (computed after the timezone offset
  from IP geolocation is applied via configTime(), not before, so it
  reflects actual local time rather than UTC).
- Selecting an aircraft on the radar now also draws a dotted bearing
  line plus a degree label from the radar center to the compass edge,
  showing which direction to actually look to spot it in the sky
  (distinct from the aircraft's own heading arrow).
- The radar info bar now shows "Empty sky for <time>" instead of the
  "tap for details" hint whenever no aircraft are currently in range
  after all filters, counting up from the last sighting.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/i18n.h

@@ -19,6 +19,9 @@ enum class StringId : uint8_t {
     SPLASH_WIFI_FAILED,
     SPLASH_GETTING_LOCATION,
     SPLASH_READY,
+    SPLASH_GREETING_MORNING,
+    SPLASH_GREETING_DAY,
+    SPLASH_GREETING_EVENING,
 
     SD_REQUIRED_LINE1,
     SD_REQUIRED_LINE2,
@@ -134,6 +137,7 @@ enum class StringId : uint8_t {
     LOGFILES_INFO_PARA2,
 
     RADAR_TAP_FOR_DETAILS,
+    RADAR_EMPTY_SKY_PREFIX,
 
     DETAIL_MODEL,
     DETAIL_TYPE,