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/activities/network/CrossPointWebServerActivity.cpp

@@ -11,6 +11,7 @@
 
 #include "MappedInputManager.h"
 #include "NetworkModeSelectionActivity.h"
+#include "SilentRestart.h"
 #include "WifiSelectionActivity.h"
 #include "activities/network/CalibreConnectActivity.h"
 #include "components/UITheme.h"
@@ -75,37 +76,17 @@ void CrossPointWebServerActivity::onExit() {
 
   state = WebServerActivityState::SHUTTING_DOWN;
 
-  // Stop the web server first (before disconnecting WiFi)
-  stopWebServer();
-
-  // Stop mDNS
-  MDNS.end();
-
-  // Stop DNS server if running (AP mode)
-  if (dnsServer) {
-    LOG_DBG("WEBACT", "Stopping DNS server...");
-    dnsServer->stop();
-    delete dnsServer;
-    dnsServer = nullptr;
+  // Skip reboot if WiFi was never activated (e.g. user backed out of mode selection).
+  if (WiFi.getMode() != WIFI_MODE_NULL) {
+    if (isApMode) {
+      WiFi.softAPdisconnect(true);
+    } else {
+      WiFi.disconnect(false);
+    }
+    delay(30);
+    silentRestart();
   }
 
-  // Brief wait for LWIP stack to flush pending packets
-  delay(50);
-
-  // Disconnect WiFi gracefully
-  if (isApMode) {
-    LOG_DBG("WEBACT", "Stopping WiFi AP...");
-    WiFi.softAPdisconnect(true);
-  } else {
-    LOG_DBG("WEBACT", "Disconnecting WiFi (graceful)...");
-    WiFi.disconnect(false);  // false = don't erase credentials, send disconnect frame
-  }
-  delay(30);  // Allow disconnect frame to be sent
-
-  LOG_DBG("WEBACT", "Setting WiFi mode OFF...");
-  WiFi.mode(WIFI_OFF);
-  delay(30);  // Allow WiFi hardware to power down
-
   LOG_DBG("WEBACT", "Free heap at onExit end: %d bytes", ESP.getFreeHeap());
 }
 
@@ -270,15 +251,6 @@ void CrossPointWebServerActivity::startWebServer() {
   }
 }
 
-void CrossPointWebServerActivity::stopWebServer() {
-  if (webServer && webServer->isRunning()) {
-    LOG_DBG("WEBACT", "Stopping web server...");
-    webServer->stop();
-    LOG_DBG("WEBACT", "Web server stopped");
-  }
-  webServer.reset();
-}
-
 void CrossPointWebServerActivity::loop() {
   // Handle different states
   if (state == WebServerActivityState::SERVER_RUNNING) {