feat(frontend): redesign the theme editor page

The editor's control panel is now a second ClickGui window skinned
live with the theme being edited, so every change shows in the tool
itself, not just the preview -- same "the section is a ClickGui
window" move the landing's ThemeStrip already uses, applied to a
working tool this time.

- Removed the three uppercase eyebrow-style section labels (the same
  generic-tell pattern already fixed on the landing page); colour
  slots are grouped into named fieldsets (Window/Modules/Text) instead
- Panel split into panel/{PanelHeader,PresetPicker,PaletteTabs,
  ExportActions}.tsx to stay under the file-size limit
- Preset chips got a real swatch (accent dot over a mini card
  gradient) -- previously same-hued presets like Classic/Blue/Azure
  all read as identical blue dots
- Copy/share buttons confirm inline ("Скопировано"/"Ссылка
  скопирована"); import/clipboard failures now surface a message
  instead of failing silently
- Rewrote themes copy to match the landing's voice

Claude-Session: https://claude.ai/code/session_01F1M1Jic1wTSn4igUENynmZ

frontend/src/features/themes/i18n/en.ts

@@ -19,12 +19,20 @@ export const themesEn = {
   },
   editor: {
     title: 'Theme editor',
-    lead: 'Build your own theme and bring it into the mod as a single file.',
+    lead: 'Start from any theme in the mod and change the colors: the menu and this panel repaint as you go.',
+    panel: 'Theme settings',
     nameLabel: 'Theme name',
-    presets: 'Ready presets',
+    presets: 'Themes from the mod',
+    palette: 'What to edit',
     colors: 'Colors',
     gradients: 'Gradients',
-    enabled: 'Gradient',
+    group: {
+      window: 'Window',
+      modules: 'Modules',
+      text: 'Text and accent',
+    },
+    alpha: '{{name}}: opacity',
+    enabledFor: 'Gradient: {{name}}',
     angle: 'Angle',
     start: 'Start',
     end: 'End',
@@ -38,12 +46,14 @@ export const themesEn = {
     copy: 'Copy for the mod',
     copied: 'Copied',
     download: 'Download .json',
-    import: 'Import from file',
-    share: 'Share link',
+    import: 'Load from file',
+    imported: 'Loaded the {{name}} theme',
+    share: 'Copy link',
     shared: 'Link copied',
-    reset: 'Reset',
+    reset: 'Reset to {{name}}',
     undo: 'Undo',
     redo: 'Redo',
-    importError: 'Could not read the theme: the file is malformed.',
+    importError: "That isn't a LoVisual theme: the file can't be read. Use a .json downloaded from this editor.",
+    clipboardError: 'The browser blocked the clipboard. Download the .json and move the file instead.',
   },
 } satisfies Translation<typeof themesRu>