Fix race condition with keyboard and Wifi entry (#204)

## Summary

* **What is the goal of this PR?** Fixes a bug -
https://github.com/daveallie/crosspoint-reader/issues/187 - where the
screen would freeze after entering a WiFi password, causing the device
to appear hung.

* **What changes are included?**
- Fixed a race condition in `WifiSelectionActivity::displayTaskLoop()`
that caused rendering of an empty screen when transitioning from the
keyboard subactivity
- Added `vTaskDelay()` when a subactivity is active to prevent CPU
starvation from a tight `continue` loop
- Added a check to skip rendering when in `PASSWORD_ENTRY` state,
allowing the state machine to properly transition to `CONNECTING` before
the display updates

## Additional Context

* **Root cause:** When the keyboard subactivity exited after password
entry, the display task would wake up and attempt to render. However,
the `state` was still `PASSWORD_ENTRY` (before `attemptConnection()`
changed it to `CONNECTING`), and since there was no render case for
`PASSWORD_ENTRY`, the display would show a cleared/empty buffer,
appearing frozen.

* **Performance implications:** The added `vTaskDelay(10)` calls when
subactivity is active or in `PASSWORD_ENTRY` state actually improve
performance by preventing CPU starvation - previously the display task
would spin in a tight loop with `continue` while a subactivity was
present.

* **Testing focus:** Test the full WiFi connection flow:
  1. Enter network selection
  2. Select a network requiring a password
  3. Enter password and press OK
  4. Verify "Connecting..." screen appears
  5. Verify connection completes and prompts to save password

src/activities/network/WifiSelectionActivity.cpp

@@ -447,7 +447,16 @@ std::string WifiSelectionActivity::getSignalStrengthIndicator(const int32_t rssi
 
 void WifiSelectionActivity::displayTaskLoop() {
   while (true) {
+    // If a subactivity is active, yield CPU time but don't render
     if (subActivity) {
+      vTaskDelay(10 / portTICK_PERIOD_MS);
+      continue;
+    }
+
+    // Don't render if we're in PASSWORD_ENTRY state - we're just transitioning
+    // from the keyboard subactivity back to the main activity
+    if (state == WifiSelectionState::PASSWORD_ENTRY) {
+      vTaskDelay(10 / portTICK_PERIOD_MS);
       continue;
     }