Fix double-space period, shift double-tap; settings; letters from system keymap

- Double-space replaced the space via a Backspace keysym, which clients
  apply after the commit: use delete_surrounding_text with the commit
- Tests use a shared FakeEngine that applies ops to a simulated field
- Double-tap shift locks at a sentence start; typing between taps doesn't
- Organization name set: config lives in ~/.config/sloposk/
- Settings (auto-capitalization, double-space period) with a ⚙ panel
- Letter rows come from the compositor's XKB keymap; other letters of the
  layout become long-press alternates; globe switches the system layout
  over org.kde.keyboard
- One JSON template for non-letter layers; stretch keys even out rows

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/tst_shiftstate.cpp

@@ -32,6 +32,37 @@ private Q_SLOTS:
         QCOMPARE(shift.mode(), Mode::Locked);
     }
 
+    void doubleTapLocksAfterAutomaticShift()
+    {
+        ShiftState shift;
+        shift.setAutomatic(true);
+        shift.press(0); // turns auto-shift off...
+        shift.release();
+        shift.press(200); // ...and the second tap locks
+        shift.release();
+        QCOMPARE(shift.mode(), Mode::Locked);
+    }
+
+    void tapAfterUnlockIsNotDoubleTap()
+    {
+        ShiftState shift;
+        shift.press(0);
+        shift.press(100); // locked
+        shift.press(1000); // unlocks
+        shift.press(1100);
+        QCOMPARE(shift.mode(), Mode::Once);
+    }
+
+    void typingBetweenTapsIsNotDoubleTap()
+    {
+        ShiftState shift;
+        shift.press(0);
+        shift.release();
+        shift.characterTyped();
+        shift.press(200);
+        QCOMPARE(shift.mode(), Mode::Once);
+    }
+
     void tapUnlocks()
     {
         ShiftState shift;