fix: silent-reboot on wifi activity exit to clear heap fragmentation (#1908)

WiFi/LWIP/netif teardown scatters long-lived allocations across the
heap, leaving ~50KB of contiguous space unrecoverable without a reboot.

Reboot the SoC on exit from any wifi-using activity to guarantee a clean
heap. An RTC_NOINIT flag survives the reboot and tells setup() to skip
the boot splash and route the user back where they came from:
  - File transfer / Calibre / OPDS / Font download -> home
  - KOReader sync -> currently-open EPUB

Activities check WiFi.getMode() before rebooting, so backing out of the
network mode menu without joining doesn't trigger a cycle. KOSync also
esp_wifi_stop()s after the sync result so the radio is off while the
user reads it; full teardown happens at the reboot.


## Additional Context

The silent reboot skips the booting splash screen - it visibly looks
like a screen refresh. This does cause a disconnection/reconnection blip
for developers actively pulling logs over serial, but `pio device
monitor` and the like successfully reconnect and feed in the early boot
serial.
as an example: 
```
[256676] [DBG] [ACT] Exiting activity: KOReaderSync
[256706] [DBG] [MAIN] Silent restart (target=reader)

ESP-ROM:esp32c3-api1-20210207
Build:Feb  7 2021
rst:0xc (RTC_SW_CPU_RST),boot:0xf (SPI_FAST_FLASH_BOOT)
Saved PC:0x403872bc
SPIWP:0xee
mode:DIO, clock div:1
load:0x3fcd72a0,len:0x990
load:0x403cbf10,len:0xac8
load:0x403ce710,len:0x4d28
entry 0x403cbf10
[22] [INF] [MAIN] Hardware detect: X4
[29] [SD] SD card detected
[43] [DBG] [CPS] Settings loaded from file
[58] [DBG] [KRS] Loaded KOReader credentials for user: jeremydk
[69] [DBG] [OPS] Loaded 1 OPDS servers from file
[69] [DBG] [UI] Using Lyra theme
[70] [DBG] [MAIN] Starting CrossPoint version 1.2.0-dev-detached-bde75787

...

[203] [DBG] [ACT] Entering activity: Reader
[211] [DBG] [EBP] Loading ePub: /Halting State - Charles Stross.epub
[221] [DBG] [BMC] Loaded cache data: 51 spine, 41 TOC entries
[246] [DBG] [CSS] Loaded 41 rules from cache
[247] [DBG] [EBP] Loaded ePub: /Halting State - Charles Stross.epub
```
---
### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _PARTIALLY_

src/main.cpp

@@ -131,6 +131,29 @@ EpdFontFamily ui12FontFamily(&ui12RegularFont, &ui12BoldFont);
 unsigned long t1 = 0;
 unsigned long t2 = 0;
 
+// Definitions for SilentRestart.h. RTC_NOINIT survives ESP.restart() but not power loss.
+RTC_NOINIT_ATTR uint32_t silentRebootMagic;
+RTC_NOINIT_ATTR uint32_t silentRebootTarget;
+constexpr uint32_t SILENT_REBOOT_MAGIC = 0xC1EAB007;
+constexpr uint32_t SILENT_REBOOT_TARGET_HOME = 0;
+constexpr uint32_t SILENT_REBOOT_TARGET_READER = 1;
+
+void silentRestart() {
+  silentRebootTarget = SILENT_REBOOT_TARGET_HOME;
+  silentRebootMagic = SILENT_REBOOT_MAGIC;
+  LOG_DBG("MAIN", "Silent restart (target=home)");
+  delay(50);
+  ESP.restart();
+}
+
+void silentRestartToReader() {
+  silentRebootTarget = SILENT_REBOOT_TARGET_READER;
+  silentRebootMagic = SILENT_REBOOT_MAGIC;
+  LOG_DBG("MAIN", "Silent restart (target=reader)");
+  delay(50);
+  ESP.restart();
+}
+
 // Verify power button press duration on wake-up from deep sleep
 // Pre-condition: isWakeupByPowerButton() == true
 void verifyPowerButtonDuration() {
@@ -238,6 +261,15 @@ void setup() {
   t1 = millis();
 
   HalSystem::begin();
+
+  // Read-and-clear so a panic later in setup() doesn't loop into silent reboot.
+  // Bound the target range too — RTC_NOINIT memory is uninitialized on cold boot.
+  const bool isSilentReboot = (silentRebootMagic == SILENT_REBOOT_MAGIC);
+  const uint32_t snapshotTarget =
+      (isSilentReboot && silentRebootTarget <= SILENT_REBOOT_TARGET_READER) ? silentRebootTarget : 0;
+  silentRebootMagic = 0;
+  silentRebootTarget = 0;
+
   gpio.begin();
   powerManager.begin();
   halTiltSensor.begin();
@@ -315,7 +347,11 @@ void setup() {
 
   setupDisplayAndFonts();
 
-  activityManager.goToBoot();
+  // First paint after silent reboot is HALF_REFRESH (SDK forces it after begin()'s
+  // panel reset); subsequent paints FAST.
+  if (!isSilentReboot) {
+    activityManager.goToBoot();
+  }
 
   APP_STATE.loadFromFile();
   RECENT_BOOKS.loadFromFile();
@@ -327,6 +363,13 @@ void setup() {
   } else if (HalSystem::isRebootFromPanic()) {
     // If we rebooted from a panic, go to crash report screen to show the panic info
     activityManager.goToCrashReport();
+  } else if (isSilentReboot && snapshotTarget == SILENT_REBOOT_TARGET_READER && !APP_STATE.openEpubPath.empty()) {
+    activityManager.goToReader(APP_STATE.openEpubPath);
+  } else if (isSilentReboot) {
+    // target == home (or reader with no open book): land on home — don't fall
+    // through to the sleep-wake "resume reader" logic, which fires on stale
+    // openEpubPath + lastSleepFromReader from a prior session.
+    activityManager.goHome();
   } else if (APP_STATE.openEpubPath.empty() || !APP_STATE.lastSleepFromReader ||
              mappedInputManager.isPressed(MappedInputManager::Button::Back) || APP_STATE.readerActivityLoadCount > 0) {
     // Boot to home screen if no book is open, last sleep was not from reader, back button is held, or reader activity