SwiftUI's .textSelection(.enabled) only permits selection within a
single Text view — drag-selecting across chat messages was impossible.
Replace the LazyVStack { MessageRow } rendering in MessageList and
ServerMessageList with a new MessageBufferView: NSViewRepresentable
wrapping NSScrollView + a non-editable NSTextView.

What you now get for free from AppKit:

- True drag-selection across any contiguous range of the buffer, across
  rows and message kinds.
- Cmd+A to select the whole buffer.
- Native Cmd+C copy of the selection as plain text.
- Native incremental Find via Cmd+F (in addition to the existing
  SwiftUI FindBar, which continues to work as a channel filter).
- Right-click context menu with Look Up / Search / Share plus our
  Copy Message / Copy Text / Copy Nickname items.

Rendering in the coordinator mirrors the prior SwiftUI output:
timestamp + sender + IRC-styled content, highlight lines get an accent
background, and a centered "── new ──" paragraph marks the first
unread message. Nick colors round-trip to NSColor via NSColor(Color).

MessageBufferView diff-appends on pure tail extensions and does a full
rebuild when options change (nick-colors toggle, timestamp format,
last-read marker, or message reordering from find-filter). Scroll
pinning follows the classic mIRC rule: if the user was at the bottom
before append, stay at the bottom; otherwise leave their position
alone so they can read history.

The dead SwiftUI views MessageRow, LineMarker, LinkPreviewView, and
the firstPreviewableURL/_linkDetector helpers are removed. Link
previews are not rendered in this build; the existing "Fetch link
previews" preference toggle has no effect until a follow-up restores
inline rendering via NSTextAttachment.

Implementation notes:
- Coordinator is @mainactor; textView and scrollView are weak refs.
- `@objc private func copyAction(_:)` is the one ObjC-adjacent line
  required to satisfy NSMenuItem's target-action contract. It is a
  private selector callback, not a new public API surface.
- `.bryggaMessageID` attributed-string key tags each message's
  paragraph range so the menu handler can recover the clicked Message.