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_keyrepeater.cpp

@@ -0,0 +1,31 @@
+#include "input/KeyRepeater.h"
+
+#include <QSignalSpy>
+#include <QTest>
+
+class TestKeyRepeater : public QObject
+{
+    Q_OBJECT
+
+private Q_SLOTS:
+    void repeatsThenSwitchesToWords()
+    {
+        KeyRepeater repeater({.initialDelayMs = 50, .intervalMs = 10, .wordModeAfterMs = 150, .wordIntervalMs = 20});
+        QSignalSpy spy(&repeater, &KeyRepeater::repeat);
+        repeater.start();
+
+        QTest::qWait(40);
+        QCOMPARE(spy.count(), 0); // still in the initial delay
+        QTRY_VERIFY(spy.count() >= 3);
+        QCOMPARE(spy.first().first().toBool(), false);
+        QTRY_VERIFY(spy.last().first().toBool()); // word mode eventually
+
+        repeater.stop();
+        const auto count = spy.count();
+        QTest::qWait(60);
+        QCOMPARE(spy.count(), count);
+    }
+};
+
+QTEST_GUILESS_MAIN(TestKeyRepeater)
+#include "tst_keyrepeater.moc"