fix(tui): replace OSC 52 jargon in /copy confirmation

When the user ran /copy successfully, Ink confirmed with:

  sent OSC52 copy sequence (terminal support required)

That reads like a protocol spec to everyone who isn't a terminal
implementer. The caveat was a historical artifact — OSC 52 wasn't
universally supported when this message was written, so the TUI
honestly couldn't guarantee the copy had landed anywhere.

Today every modern terminal (including the dashboard's embedded
xterm.js) handles OSC 52 reliably. Say what the user actually wants
to know — that it copied, and how much — matching the message the
TUI already uses for selection copy:

  copied 1482 chars

ui-tui/src/app/slash/commands/core.ts

@@ -219,7 +219,7 @@ export const coreCommands: SlashCommand[] = [
       }
 
       writeOsc52Clipboard(target.text)
-      sys('sent OSC52 copy sequence (terminal support required)')
+      sys(`copied ${target.text.length} chars`)
     }
   },