refactor(client): adopt copyToClipboard helper at raw Clipboard.setData sites

[NC1107]

Summary

Migrates 8 raw Clipboard.setData(...) + ToastService.show(..., type: ToastType.success) pairs to the existing copyToClipboard(context, text, successMessage: ...) helper in services/clipboard_service.dart. Toast wording is preserved verbatim; the helper API is unchanged.

Migrated sites

• screens/settings/account_section.dart:343 — _copyInviteLink (invite URL).
• screens/settings/account_section.dart:412 — QR-code dialog "Copy invite link" button (uses dialogContext).
• screens/settings/about_section.dart:768 — bulk copy of debug-log entries.
• screens/settings/about_section.dart:919 — copy a single debug-log entry.
• screens/group_info_screen/parts/invite_section.dart:31 — copy freshly-minted group invite URL.
• widgets/connection_status_badge.dart:464 — diagnostics popover copy.
• widgets/message_item.dart:1441 — _copyMessageText (message body or media URL).
• widgets/message_item.dart:1451 — _copyMessageId.

Skipped sites and why

• screens/settings/data_storage_section.dart:84 — the surrounding ToastService.show omits the type: arg, so it defaults to ToastType.info. The helper hard-codes success, so migrating would change the toast type.
• widgets/image_gallery_viewer.dart:145 — fallback path uses ToastType.info ("Save not supported here. Link copied.").
• widgets/message_item.dart:313, 356 — both are fallback paths that surface ToastType.info ("Save not supported here yet. Link copied.", "Image copy not supported, link copied").
• widgets/message/media_content.dart:307 — fallback path uses ToastType.info.
• widgets/chat_input_bar/parts/keyboard_handling.dart:77 — Ctrl+C / Ctrl+X keyboard handler; intentionally has no toast and isn't a "copy" UX surface.

Behind the scenes

• Drops now-unused flutter/services.dart and toast_service.dart imports from files that no longer reference them.
• flutter analyze --fatal-infos is clean; dart format is a no-op on the touched files.
• Affected widget tests pass (account_section_test, about_section_test, connection_status_badge_test, message_item_test — 51/51).

Test plan

• Settings → Account → "Copy invite link" still shows the existing toast.
• Settings → About → Debug logs → copy all + copy single still show their toasts.
• Group info → generate invite link still shows the existing toast.
• Connection status popover → diagnostics copy still shows the existing toast.
• Message context menu → copy message + copy ID still show their toasts.