feat: add shift lock to KeyboardEntryActivity (#513)

## Summary

* **What is the goal of this PR?** (e.g., Implements the new feature for
file uploading.)
Add shift lock to KeyboardEntryActivity


https://github.com/user-attachments/assets/00973866-6b87-4d5b-a3bf-6f4f85a5e0a6

(Apologies for the graininess of the video - I was struggling to get it
below 10mb)

* **What changes are included?**
  * Relax shift disable criteria to include any character
* Add third shift option `LOCK` which is not disabled on character
entry.


## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
  specific areas to focus on).

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src/activities/util/KeyboardEntryActivity.h

@@ -70,7 +70,7 @@ class KeyboardEntryActivity : public Activity {
   // Keyboard state
   int selectedRow = 0;
   int selectedCol = 0;
-  bool shiftActive = false;
+  int shiftState = 0;  // 0 = lower case, 1 = upper case, 2 = shift lock)
 
   // Callbacks
   OnCompleteCallback onComplete;
@@ -81,6 +81,7 @@ class KeyboardEntryActivity : public Activity {
   static constexpr int KEYS_PER_ROW = 13;  // Max keys per row (rows 0 and 1 have 13 keys)
   static const char* const keyboard[NUM_ROWS];
   static const char* const keyboardShift[NUM_ROWS];
+  static const char* const shiftString[3];
 
   // Special key positions (bottom row)
   static constexpr int SPECIAL_ROW = 4;