Restructure into src/ layout under pygui package

Move all application source under src/pygui/ and rewire imports,
build config, and QML module path to match.
- Relocate backend/, serialcomm/, and the ISC QML module into
  src/pygui/; convert main.py into pygui/__main__.py with a main()
  entry point (run via `python -m pygui` or the new `isc` script)
- Rewrite absolute imports: backend.* -> pygui.backend.*,
  serialcomm.* -> pygui.serialcomm.* (source + tests)
- Move app icons (isc.ico/icns) into packaging/
- Update README and ISC.qmlproject to the new paths

src/pygui/ISC/Theme.qml

@@ -0,0 +1,136 @@
+pragma Singleton
+
+import QtQuick
+
+// =============================================================================
+//  Theme — single source of truth for colors and sizing
+// =============================================================================
+//  Sections:
+//    1.  Mode flag
+//    2.  Tone palette          (raw tones; everything else derives from these)
+//    3.  Surfaces              (page / card / panel / workspace backgrounds)
+//    4.  Borders
+//    5.  Text                  (headings, body, panel titles)
+//    6.  Controls              (fields, buttons, checkbox)
+//    7.  Tracks                (pill toggle + scrollbar)
+//    8.  Tabs                  (footer tab bar)
+//    9.  Side rail
+//   10.  Status                (success / warning / error)
+//   11.  Geometry              (radius / border width / spacing / sizing)
+// =============================================================================
+
+QtObject {
+    // ── 1. Mode ──────────────────────────────────────────────────────────────
+    property bool isDarkMode: false
+
+    // ── 2. Tone palette (base values; consume via semantic tokens below) ─────
+    readonly property color tone100: isDarkMode ? "#111111" : "#FAFAFA"
+    readonly property color tone200: isDarkMode ? "#1A1A1A" : "#F2F2F2"
+    readonly property color tone300: isDarkMode ? "#242424" : "#E8E8E8"
+    readonly property color toneText: isDarkMode ? "#F2F2F2" : "#111111"
+    readonly property color toneMute: isDarkMode ? "#A8A8A8" : "#8A8A8A"
+    readonly property color toneBorder: isDarkMode ? "#2B2B2B" : "#D8D8D8"
+
+    // ── 3. Surfaces ──────────────────────────────────────────────────────────
+    readonly property color pageBackground: tone100
+    readonly property color cardBackground: tone200
+    readonly property color panelBackground: tone200
+    readonly property color workspaceBackground: tone200
+    readonly property color responseBackground: tone200
+    readonly property color subtleSectionBackground: tone300
+
+    // ── 4. Borders ───────────────────────────────────────────────────────────
+    readonly property color cardBorder: toneBorder
+    readonly property color responseBorder: toneBorder
+    readonly property color workspaceBorder: toneBorder
+    readonly property color innerFrameBorder: toneBorder
+    readonly property color outerFrameBorder: isDarkMode ? "#343434" : "#CECECE"
+    readonly property color softBorder: isDarkMode ? "#222222" : "#E2E2E2"
+
+    // ── 5. Text ──────────────────────────────────────────────────────────────
+    readonly property color headingColor: toneText
+    readonly property color bodyColor: toneMute
+    readonly property color subheadingColor: toneMute
+    readonly property color panelTitleText: toneMute
+    readonly property color disabledText: toneMute
+
+    // ── 6. Controls ──────────────────────────────────────────────────────────
+    // Fields
+    readonly property color fieldBackground: tone100
+    readonly property color fieldText: toneText
+    readonly property color fieldPlaceholder: toneMute
+    readonly property color fieldBorder: toneBorder
+    readonly property color fieldBorderFocus: toneText
+    // Neutral buttons
+    readonly property color buttonNeutralBackground: tone300
+    readonly property color buttonNeutralHover: isDarkMode ? "#303030" : "#DDDDDD"
+    readonly property color buttonNeutralPressed: tone100
+    readonly property color buttonNeutralText: toneText
+    readonly property color buttonPressed: tone300
+    readonly property color primaryAccent: toneText
+    // Checkbox
+    readonly property color checkboxText: toneText
+
+    // ── 7. Tracks (pill toggle, scrollbar thumb) ─────────────────────────────
+    readonly property color trackBackground: isDarkMode ? "#25252B" : "#D1D5DB"
+
+    // ── 8. Tabs (footer tab bar) ─────────────────────────────────────────────
+    readonly property color tabBarBackground: tone200
+    readonly property color tabBackground: "transparent"
+    readonly property color tabActiveBackground: "transparent"
+    readonly property color tabHoverBackground: tone300
+    readonly property color tabBorder: "transparent"
+    readonly property color tabText: toneMute
+    readonly property color tabActiveText: toneText
+
+    // ── 9. Side rail ─────────────────────────────────────────────────────────
+    readonly property color sideRailBackground: tone200
+    readonly property color sideActiveBackground: tone300
+
+    // ── 10. Status ───────────────────────────────────────────────────────────
+    readonly property color statusSuccessColor: isDarkMode ? "#63D471" : "#2E9E44"
+    readonly property color statusWarningColor: isDarkMode ? "#F5C15C" : "#C88A18"
+    readonly property color statusErrorColor: isDarkMode ? "#FF6B6B" : "#D64545"
+
+    // -- 10b. Sensor bands (wafer map dots)
+    readonly property color sensorInRange: statusSuccessColor
+    readonly property color sensorHigh: statusErrorColor
+    readonly property color sensorlow: isDarkMode ? "#589DF5" : "#2F6FE0"
+    readonly property color waferRingColor: toneBorder
+    readonly property color waferAxisColor: softBorder
+
+    // ── 11. Geometry ─────────────────────────────────────────────────────────
+    // Radius
+    readonly property int radiusXs: 4   // fields, tight elements
+    readonly property int radiusSm: 6   // buttons
+    readonly property int radiusMd: 10  // cards, group boxes
+    readonly property int radiusLg: 14  // large panels / dialogs
+    // Border width
+    readonly property int borderThin: 1
+    readonly property int borderStrong: 2
+    // Main panel layout
+    readonly property int mainAreaPadding: 10
+    readonly property int rightPaneGap: 6
+    readonly property int panelPadding: 12
+    // Side rail layout
+    readonly property int sideRailWidth: 190
+    readonly property int sideRailSpacing: 14
+    readonly property int sideButtonHeight: 146
+    readonly property int sideButtonMinHeight: 88
+    // Tab bar layout
+    readonly property int tabBarHeight: 34
+    readonly property int tabBarPadding: 6
+    readonly property int tabSpacing: 2
+    readonly property int tabRadius: 2
+    readonly property int tabFontSize: 12
+    readonly property int tabButtonMinWidth: 80
+    // Settings page layout
+    readonly property int settingsPanelMaxWidth: 760
+    readonly property int settingsOuterMargin: 40
+    readonly property int settingsSectionSpacing: 20
+    readonly property int settingsRowSpacing: 16
+    readonly property int settingsGridSpacing: 12
+    readonly property int settingsButtonHeight: 40
+    readonly property int settingsActionWidth: 200
+    readonly property int settingsFieldMinWidth: 160
+}