Add HTML and PDF conversation export

Top item on the roadmap ranking from 2026-07-27 — multi-modal export
alongside the existing Markdown/JSON. New ConversationExportService
consolidates the two previously-duplicated Markdown builders
(ChatViewModel and ConversationListView had separate copies of the
same **User**/**Assistant** + --- format) and adds:

- A hand-rolled Markdown->HTML renderer scoped to what actually shows
  up in chat messages (headers, bold/italic, inline code, fenced code
  blocks, lists, blockquotes, links, horizontal rules) rather than
  full CommonMark/GFM — no existing markdown-to-HTML utility existed
  in the codebase, and swift-markdown-ui is SwiftUI-view-only with no
  HTML-string export API. Content is HTML-escaped before any markdown
  substitution so example code containing "<div>" etc renders as
  visible text, not live markup.
- PDF via an offscreen WKWebView loading that same HTML and calling
  the official createPDF(configuration:) API (macOS 11+) — no new
  project/framework linkage needed, WebKit is a system framework.

Wired into every place Markdown export already existed: File menu
(Export as HTML.../PDF...), /export slash command (now md|html|pdf|json),
and a new Export submenu (Markdown/HTML/PDF) on each conversation row's
context menu in the advanced conversation list, replacing the old
single-format swipe-only export. Help docs and InputBar autocomplete
updated to match.

oAI/oAIApp.swift

@@ -124,6 +124,20 @@ struct oAIApp: App {
                     chatViewModel.exportConversation(format: "md", filename: "\(safe).md")
                 }
                 .disabled(chatViewModel.messages.filter { $0.role != .system }.isEmpty)
+
+                Button("Export as HTML…") {
+                    let name = chatViewModel.currentConversationName ?? "conversation"
+                    let safe = name.components(separatedBy: .whitespacesAndNewlines).joined(separator: "-")
+                    chatViewModel.exportConversation(format: "html", filename: "\(safe).html")
+                }
+                .disabled(chatViewModel.messages.filter { $0.role != .system }.isEmpty)
+
+                Button("Export as PDF…") {
+                    let name = chatViewModel.currentConversationName ?? "conversation"
+                    let safe = name.components(separatedBy: .whitespacesAndNewlines).joined(separator: "-")
+                    chatViewModel.exportConversation(format: "pdf", filename: "\(safe).pdf")
+                }
+                .disabled(chatViewModel.messages.filter { $0.role != .system }.isEmpty)
             }
 
             // ── Chat menu ─────────────────────────────────────────────────