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.cpp

@@ -0,0 +1,119 @@
+#include "theme.h"
+
+namespace rcx {
+
+// ── Field table for DRY serialization ──
+
+struct ColorField { const char* key; QColor Theme::*ptr; };
+
+static const ColorField kFields[] = {
+    {"background",    &Theme::background},
+    {"backgroundAlt", &Theme::backgroundAlt},
+    {"surface",       &Theme::surface},
+    {"border",        &Theme::border},
+    {"button",        &Theme::button},
+    {"text",          &Theme::text},
+    {"textDim",       &Theme::textDim},
+    {"textMuted",     &Theme::textMuted},
+    {"textFaint",     &Theme::textFaint},
+    {"hover",         &Theme::hover},
+    {"selected",      &Theme::selected},
+    {"selection",     &Theme::selection},
+    {"syntaxKeyword", &Theme::syntaxKeyword},
+    {"syntaxNumber",  &Theme::syntaxNumber},
+    {"syntaxString",  &Theme::syntaxString},
+    {"syntaxComment", &Theme::syntaxComment},
+    {"syntaxPreproc", &Theme::syntaxPreproc},
+    {"syntaxType",    &Theme::syntaxType},
+    {"indHoverSpan",  &Theme::indHoverSpan},
+    {"indCmdPill",    &Theme::indCmdPill},
+    {"indDataChanged",&Theme::indDataChanged},
+    {"indHintGreen",  &Theme::indHintGreen},
+    {"markerPtr",     &Theme::markerPtr},
+    {"markerCycle",   &Theme::markerCycle},
+    {"markerError",   &Theme::markerError},
+};
+
+QJsonObject Theme::toJson() const {
+    QJsonObject o;
+    o["name"] = name;
+    for (const auto& f : kFields)
+        o[f.key] = (this->*f.ptr).name();
+    return o;
+}
+
+Theme Theme::fromJson(const QJsonObject& o) {
+    Theme t = reclassDark();
+    t.name = o["name"].toString(t.name);
+    for (const auto& f : kFields) {
+        if (o.contains(f.key))
+            t.*f.ptr = QColor(o[f.key].toString());
+    }
+    return t;
+}
+
+// ── Built-in themes ──
+
+Theme Theme::reclassDark() {
+    Theme t;
+    t.name          = "Reclass Dark";
+    t.background    = QColor("#1e1e1e");
+    t.backgroundAlt = QColor("#252526");
+    t.surface       = QColor("#2a2d2e");
+    t.border        = QColor("#3c3c3c");
+    t.button        = QColor("#333333");
+    t.text          = QColor("#d4d4d4");
+    t.textDim       = QColor("#858585");
+    t.textMuted     = QColor("#585858");
+    t.textFaint     = QColor("#505050");
+    t.hover         = QColor("#2b2b2b");
+    t.selected      = QColor("#232323");
+    t.selection     = QColor("#2b2b2b");
+    t.syntaxKeyword = QColor("#569cd6");
+    t.syntaxNumber  = QColor("#b5cea8");
+    t.syntaxString  = QColor("#ce9178");
+    t.syntaxComment = QColor("#6a9955");
+    t.syntaxPreproc = QColor("#c586c0");
+    t.syntaxType    = QColor("#4EC9B0");
+    t.indHoverSpan  = QColor("#E6B450");
+    t.indCmdPill    = QColor("#2a2a2a");
+    t.indDataChanged= QColor("#8fbc7a");
+    t.indHintGreen  = QColor("#5a8248");
+    t.markerPtr     = QColor("#f44747");
+    t.markerCycle   = QColor("#e5a00d");
+    t.markerError   = QColor("#7a2e2e");
+    return t;
+}
+
+Theme Theme::warm() {
+    Theme t;
+    t.name          = "Warm";
+    t.background    = QColor("#212121");
+    t.backgroundAlt = QColor("#2a2a2a");
+    t.surface       = QColor("#2a2a2a");
+    t.border        = QColor("#373737");
+    t.button        = QColor("#373737");
+    t.text          = QColor("#AAA99F");
+    t.textDim       = QColor("#7a7a6e");
+    t.textMuted     = QColor("#555550");
+    t.textFaint     = QColor("#464646");
+    t.hover         = QColor("#373737");
+    t.selected      = QColor("#2d2d2d");
+    t.selection     = QColor("#21213A");
+    t.syntaxKeyword = QColor("#AA9565");
+    t.syntaxNumber  = QColor("#AAA98C");
+    t.syntaxString  = QColor("#6B3B21");
+    t.syntaxComment = QColor("#464646");
+    t.syntaxPreproc = QColor("#AA9565");
+    t.syntaxType    = QColor("#6B959F");
+    t.indHoverSpan  = QColor("#AA9565");
+    t.indCmdPill    = QColor("#2a2a2a");
+    t.indDataChanged= QColor("#6B959F");
+    t.indHintGreen  = QColor("#464646");
+    t.markerPtr     = QColor("#6B3B21");
+    t.markerCycle   = QColor("#AA9565");
+    t.markerError   = QColor("#3C2121");
+    return t;
+}
+
+} // namespace rcx