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/__tests__/platform.test.ts

@@ -51,6 +51,12 @@ describe('isCopyShortcut', () => {
 
     expect(isCopyShortcut({ ctrl: false, meta: true, super: false }, 'c', {})).toBe(false)
   })
+
+  it('accepts the VS Code/Cursor forwarded Cmd+C copy sequence on macOS', async () => {
+    const { isCopyShortcut } = await importPlatform('darwin')
+
+    expect(isCopyShortcut({ ctrl: true, meta: false, super: true }, 'c', {})).toBe(true)
+  })
 })
 
 describe('isVoiceToggleKey', () => {