feat: warn when a PDF export replaces characters it cannot draw

The hand-written PDF writer uses the standard Courier fonts
(WinAnsiEncoding), so any character outside CP1252 - a Polish, Greek,
Cyrillic or CJK script - was silently written as '?'. Render now
collects the characters it had to replace and returns them, and the
export dialog lists them in a warning once the file is written.

Embedding a TrueType font (e.g. Courier Prime) with a CID/ToUnicode
map would fix the root cause; the warning keeps the zero-dependency
trade-off honest in the meantime.

frontend/src/backend.ts

@@ -144,13 +144,21 @@ export interface ExportOptions {
   revisions: boolean
 }
 
+/** A written PDF export. */
+export interface ExportResult {
+  /** Empty when the user cancelled the dialog. */
+  path: string
+  /** Characters the standard PDF fonts cannot draw, written as "?" in the file. */
+  replaced?: string[]
+}
+
 interface GoApp {
   NewDocument(): Promise<ScriptDocument>
   InitialFile(): Promise<OpenResult | null>
   Open(): Promise<OpenResult | null>
   Save(doc: ScriptDocument, path: string): Promise<string>
   SaveAs(doc: ScriptDocument, current: string): Promise<string>
-  ExportPDF(doc: ScriptDocument, opts: ExportOptions, current: string): Promise<string>
+  ExportPDF(doc: ScriptDocument, opts: ExportOptions, current: string): Promise<ExportResult>
   ExportFDX(doc: ScriptDocument, current: string): Promise<string>
   Paginate(doc: ScriptDocument): Promise<LayoutInfo>
   ParseFountain(text: string): Promise<Element[] | null>

frontend/src/main.ts

@@ -947,7 +947,15 @@ const actions: Record<string, () => unknown> = {
         renderPaper()
         settingsChanged()
       }
-      await api.ExportPDF(script(), chosen, app.path).catch(reportError)
+      const res = await api.ExportPDF(script(), chosen, app.path).catch(reportError)
+      if (res?.replaced?.length) {
+        const list = res.replaced.map((c) => `"${c}"`).join(' ')
+        await messageBox({
+          title: 'Characters replaced in the PDF',
+          text: `The standard PDF fonts cannot draw these characters: ${list}. Each one is written as "?" in the exported file.`,
+          ok: 'OK',
+        })
+      }
     }
     dialog.showModal()
   },