fix: Kasten-CSS des Pandoc-Filters wirkt auch mit -H und in EPUB-Kapiteln

- CSS in pandoc/callouts.css (eine Quelle), vom Filter neben sich gelesen
- HTML: <style> am Dokumentanfang statt header-includes, das ein -H auf der
  Kommandozeile ersetzt hat (PDF über weasyprint mit Druck-CSS: Kästen ohne Rahmen)
- EPUB: Kasten-CSS als Stylesheet aller Kapitel (zusammen mit Pandocs
  Standard-epub.css oder zu eigenen css:-Angaben); vorher nur auf der Titelseite

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pandoc/callouts.lua

@@ -11,7 +11,9 @@ epub2md writes boxes as GitHub alerts / Obsidian callouts:
 Pandoc < 3.1.10 does not know this syntax and prints "[!TIP]" literally. This filter turns such
 block quotes into real boxes:
 
-  * HTML, EPUB:   <div class="callout callout-tip"> with a bold label and a small stylesheet
+  * HTML, EPUB:   <div class="callout callout-tip"> with a bold label, styled by callouts.css
+                  (next to this filter; HTML: <style> at the start of the body, so -H cannot drop it;
+                  EPUB: added to the book's stylesheets together with Pandoc's default epub.css)
   * other formats (LaTeX/PDF, DOCX, ODT …): block quote with a bold label "Tipp: Title"
 
 It also handles <div class="callout …"> coming back from HTML (md -> html -> pdf route) and styles
@@ -26,17 +28,40 @@ local LABELS = {
   en = { NOTE = "Note", TIP = "Tip", IMPORTANT = "Important", WARNING = "Warning", CAUTION = "Caution" },
 }
 
-local CSS = [[
-<style>
-.callout, .box { border: 1px solid #c8ccd2; border-left: 0.35em solid #4a7bd0; border-radius: 0.3em;
-  background: #f5f8fc; padding: 0.4em 1em; margin: 1em 0; }
-.callout > p:first-child { margin-top: 0.4em; }
-.callout-tip { border-left-color: #2e9a4f; background: #f3faf5; }
-.callout-important { border-left-color: #8250df; background: #f8f5fe; }
-.callout-warning { border-left-color: #c58a00; background: #fffaf0; }
-.callout-caution { border-left-color: #cf222e; background: #fff5f5; }
-</style>
-]]
+-- callouts.css next to this filter is the single source of the box styles.
+local function read_file(path)
+  local fh = io.open(path, "r")
+  if not fh then
+    return nil
+  end
+  local text = fh:read("a")
+  fh:close()
+  return text
+end
+
+local CSS_PATH = (PANDOC_SCRIPT_FILE and PANDOC_SCRIPT_FILE:match("^(.*[/\\])") or "") .. "callouts.css"
+local CSS = read_file(CSS_PATH)
+if not CSS then
+  io.stderr:write("callouts.lua: " .. CSS_PATH .. " nicht gefunden – Kästen ohne Rahmen\n")
+end
+
+-- EPUB: a stylesheet in metadata `css` replaces Pandoc's default epub.css, so combine both in a temp file.
+local function epub_stylesheet()
+  local default = ""
+  local pipe = io.popen("pandoc --print-default-data-file epub.css 2>/dev/null")
+  if pipe then
+    default = pipe:read("a") or ""
+    pipe:close()
+  end
+  local path = os.tmpname() -- small temp file; Pandoc reads it after the filter has finished
+  local fh = io.open(path, "w")
+  if not fh then
+    return nil
+  end
+  fh:write(default, "\n", CSS)
+  fh:close()
+  return path
+end
 
 local lang = "de"
 
@@ -79,15 +104,23 @@ local function Meta(meta)
       lang = l
     end
   end
-  if is_html() then
-    local includes = meta["header-includes"]
-    if includes == nil then
-      includes = pandoc.MetaList({})
-    elseif includes.t ~= "MetaList" then
-      includes = pandoc.MetaList({ includes })
+  if CSS and FORMAT:match("epub") then
+    local css = meta.css
+    local path = nil
+    if css == nil then -- no own stylesheet: default epub.css + ours; otherwise just add ours
+      path = epub_stylesheet()
     end
-    includes:insert(pandoc.MetaBlocks({ pandoc.RawBlock("html", CSS) }))
-    meta["header-includes"] = includes
+    if css == nil then
+      css = pandoc.List({})
+    elseif pandoc.utils.type(css) ~= "List" then
+      css = pandoc.List({ css })
+    end
+    if path then
+      css:insert(pandoc.MetaString(path))
+    else
+      css:insert(pandoc.MetaString(CSS_PATH))
+    end
+    meta.css = css
   end
   return meta
 end
@@ -126,7 +159,16 @@ local function Div(div)
   return pandoc.BlockQuote(div.content)
 end
 
+-- HTML: the stylesheet goes into the body, because a command-line -H replaces header-includes from a filter.
+local function Pandoc(doc)
+  if CSS and FORMAT:match("html") and not FORMAT:match("epub") then
+    doc.blocks:insert(1, pandoc.RawBlock("html", "<style>\n" .. CSS .. "</style>"))
+  end
+  return doc
+end
+
 return {
   { Meta = Meta },
   { BlockQuote = BlockQuote, Div = Div },
+  { Pandoc = Pandoc },
 }