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>

src/engine/InputEngine.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <QString>
+#include <qnamespace.h>
 
 // Turns what the keyboard produces into text operations.
 // DirectEngine passes them straight through; prediction and swipe engines
@@ -11,5 +12,5 @@ public:
     virtual ~InputEngine() = default;
 
     virtual void commitText(const QString &text) = 0;
-    virtual void sendKeysym(quint32 keysym) = 0;
+    virtual void sendKeysym(quint32 keysym, Qt::KeyboardModifiers modifiers = Qt::NoModifier) = 0;
 };