Make chat content selectable and add Copy context menu

[buggerman]

Summary

• Make every message kind selectable, not just privmsg/notice bodies — timestamps, sender nicks, actions, joins/parts/quits/nicks/kicks, topics, mode changes, and server numerics now all respond to drag-select and ⌘C.
• Add a right-click context menu on each message row with Copy Message, Copy Text, and Copy Nickname.

Changes

• Sources/BryggaCore/IRC/IRCFormatting.swift — new public stripControlCodes(_:) helper. Strips bold / italic / underline / strikethrough / reverse / reset / monospace / color (^K<fg>[,<bg>]) control bytes, preserving the surrounding text. Additive; no existing API changed.
• Sources/Brygga/Views/ContentView.swift —
  • import AppKit for NSPasteboard.
  • .textSelection(.enabled) hoisted to the LazyVStack inside MessageList and ServerMessageList so selection works across every row kind in every list (main chat, server console, detached window).
  • Redundant per-Text .textSelection modifiers in MessageRow removed.
  • MessageRow.rowBody gains a .contextMenu with the three Copy items and a private copy(_:) pasteboard helper plus a plainLogLine computed property. Copy Message emits [HH:mm] <nick> body for privmsg, [HH:mm] -nick- body for notice, and [HH:mm] * nick body for action / presence kinds.
• Tests/IRCFormattingTests.swift — 5 new tests covering bold/italic/underline, color sequences with fg+bg, literal comma after ^K<digit> (ambiguous-separator case), reset + reverse + strikethrough, and plain-text identity.

Test plan

• `swift build` — passes
• `swift test` — 88 tests pass (was 83, +5 new)
• `swiftformat --lint .` — clean
• Manual (macOS 15 / Liquid Glass on 26):
  • Drag across a run of messages of mixed kinds; ⌘C; paste into TextEdit; confirm rows render as plain-text log lines.
  • Right-click any privmsg / notice / action / join row → verify Copy Message, Copy Text, Copy Nickname each place the expected string on the clipboard.
  • Click a URL in a message — link still opens (no regression from the selection modifier).
  • Repeat in the server console tab and in a detached channel window (⌘⇧D).

Risk / rollback

Low. Purely additive on the selection side; no protocol, persistence, or threading changes. Revert the commit if the context menu conflicts with link-tap behaviour on any macOS version we care about.

Known limitation addressed in a follow-up: SwiftUI's `.textSelection` only supports per-`Text` selection — drag-selection does not cross row boundaries. Proper mIRC-style multi-line copy lands in a follow-up PR that backs `MessageList` with an `NSViewRepresentable` wrapping `NSScrollView` + `NSTextView`.