Add theme system with Reclass Dark and Warm built-in themes

Replaces ~40 hardcoded color values with 27 semantic color roles.
Adds ThemeManager singleton, theme editor dialog, View > Theme menu,
JSON persistence for user themes, and fixes inline edit selection
color from blue #264f78 to #2b2b2b.

src/themes/theme.h

@@ -0,0 +1,55 @@
+#pragma once
+#include <QColor>
+#include <QString>
+#include <QJsonObject>
+
+namespace rcx {
+
+struct Theme {
+    QString name;
+
+    // ── Chrome ──
+    QColor background;      // editor bg, margin bg, window
+    QColor backgroundAlt;   // panels, tab selected, tooltips
+    QColor surface;         // alternateBase
+    QColor border;          // separators, menu borders
+    QColor button;          // button bg
+
+    // ── Text ──
+    QColor text;            // primary text, caret, identifiers
+    QColor textDim;         // margin fg, status bar
+    QColor textMuted;       // inactive tab, disabled menu
+    QColor textFaint;       // margin dim, hex dim
+
+    // ── Interactive ──
+    QColor hover;           // row hover, tab hover, menu hover
+    QColor selected;        // row selection highlight
+    QColor selection;       // text selection background
+
+    // ── Syntax ──
+    QColor syntaxKeyword;
+    QColor syntaxNumber;
+    QColor syntaxString;
+    QColor syntaxComment;
+    QColor syntaxPreproc;
+    QColor syntaxType;      // custom types / GlobalClass
+
+    // ── Indicators ──
+    QColor indHoverSpan;    // hover link text
+    QColor indCmdPill;      // command row pill bg
+    QColor indDataChanged;  // changed data values
+    QColor indHintGreen;    // comment/hint text
+
+    // ── Markers ──
+    QColor markerPtr;       // null pointer
+    QColor markerCycle;     // cycle detection
+    QColor markerError;     // error row bg
+
+    QJsonObject toJson() const;
+    static Theme fromJson(const QJsonObject& obj);
+
+    static Theme reclassDark();
+    static Theme warm();
+};
+
+} // namespace rcx