Text rules: auto-capitalization, double-space period, backspace repeat

- TextContext: field purpose/hints and text before the cursor, reported by
  the backend (KWin content_type + surrounding_text)
- TextRules: sentence-start and double-space heuristics, deletion helpers
- KeyRepeater: held backspace repeats, then deletes whole words (Ctrl+BS)
- Controller keeps a best-guess of the text before the cursor so rules work
  instantly and in fields that don't report surrounding text
- No auto-capitalization or smart punctuation in password/URL/email fields

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

tests/tst_keyboardsurface.cpp

@@ -25,10 +25,15 @@ class FakeEngine : public InputEngine
 {
 public:
     void commitText(const QString &text) override { output += text; }
-    void sendKeysym(quint32 keysym) override { keysyms.append(keysym); }
+    void sendKeysym(quint32 keysym, Qt::KeyboardModifiers modifiers) override
+    {
+        keysyms.append(keysym);
+        lastModifiers = modifiers;
+    }
 
     QString output;
     QList<quint32> keysyms;
+    Qt::KeyboardModifiers lastModifiers;
 };
 
 constexpr QPoint KeyboardOffset(0, 100); // room above for popups