feat: add silent NTP time sync on boot via saved WiFi credentials

New "Auto Sync on Boot" toggle in Clock Settings. When enabled, a
background FreeRTOS task scans for saved WiFi networks at boot,
connects, syncs time via NTP, then tears down WiFi — all without
blocking boot or requiring user interaction. If no saved network is
found after two scan attempts (with a 3-second retry gap), it bails
silently.

Conflict guards (BootNtpSync::cancel()) added to all WiFi-using
activities so the background task cleans up before any user-initiated
WiFi flow. Also fixes clock not appearing in the header until a button
press by detecting the invalid→valid time transition after NTP sync.

Made-with: Cursor

lib/I18n/I18nKeys.h

@@ -400,6 +400,7 @@ enum class StrId : uint16_t {
   STR_INDEXING_STATUS_ICON,
   STR_SYNC_CLOCK,
   STR_TIME_SYNCED,
+  STR_AUTO_NTP_SYNC,
   STR_MANAGE_BOOK,
   STR_ARCHIVE_BOOK,
   STR_UNARCHIVE_BOOK,