fix(tui): restore macOS copy behavior and theme polish (#17131)

This PR groups the TUI fixes that restore macOS Terminal usability and clean up the theme/composer regressions:

- copy transcript selections on macOS drag-release so Terminal.app users can copy while mouse tracking is enabled
- copy composer selections on macOS drag-release; composer selection is internal to TextInput and does not use the global Ink selection bus
- keep IDE Cmd+C forwarding setup macOS-only, and make keybinding conflict checks respect simple when-clause overlap/negation
- force truecolor before chalk initializes (unless NO_COLOR / FORCE_COLOR / HERMES_TUI_TRUECOLOR opt-outs apply) so the default banner keeps its gold/amber/bronze gradient in Terminal.app
- move TUI surfaces onto semantic theme tokens and preserve skin prompt symbols as bare tokens with renderer-owned spacing
- render focused placeholders as dim hint text in TTY mode instead of inverse/selected-looking synthetic cursor text

ui-tui/src/components/messageLine.tsx

@@ -80,13 +80,13 @@ export const MessageLine = memo(function MessageLine({
     const preview = compactPreview(stripped, maxChars) || '(empty tool result)'
 
     return (
-      <Box alignSelf="flex-start" borderColor={t.color.dim} borderStyle="round" marginLeft={3} paddingX={1}>
+      <Box alignSelf="flex-start" borderColor={t.color.muted} borderStyle="round" marginLeft={3} paddingX={1}>
         {hasAnsi(msg.text) ? (
           <Text wrap="truncate-end">
             <Ansi>{msg.text}</Ansi>
           </Text>
         ) : (
-          <Text color={t.color.dim} wrap="truncate-end">
+          <Text color={t.color.muted} wrap="truncate-end">
             {preview}
           </Text>
         )}
@@ -101,7 +101,7 @@ export const MessageLine = memo(function MessageLine({
 
   const content = (() => {
     if (msg.kind === 'slash') {
-      return <Text color={t.color.dim}>{msg.text}</Text>
+      return <Text color={t.color.muted}>{msg.text}</Text>
     }
 
     if (msg.role !== 'user' && hasAnsi(msg.text)) {
@@ -125,7 +125,7 @@ export const MessageLine = memo(function MessageLine({
       return (
         <Text color={body}>
           {head}
-          <Text color={t.color.dim} dimColor>
+          <Text color={t.color.muted} dimColor>
             [long message]
           </Text>
           {rest.join('')}