htmlToPlainText(): Format paragraphs with indentation (#7027)

Cf. <https://www.mobileread.com/forums/showthread.php?p=4072308#post4072308>.

frontend/util.lua

@@ -1002,7 +1002,7 @@ This may fail on complex HTML (with styles, scripts, comments), but should be fi
 function util.htmlToPlainText(text)
     -- Replace <br> and <p> with \n
     text = text:gsub("%s*<%s*br%s*/?>%s*", "\n") -- <br> and <br/>
-    text = text:gsub("%s*<%s*p%s*>%s*", "\n") -- <p>
+    text = text:gsub("%s*<%s*p%s*>%s*", "\n&nbsp;&nbsp;&nbsp;&nbsp;") -- <p>
     text = text:gsub("%s*</%s*p%s*>%s*", "\n") -- </p>
     text = text:gsub("%s*<%s*p%s*/>%s*", "\n") -- standalone <p/>
     -- Remove all HTML tags
@@ -1012,6 +1012,8 @@ function util.htmlToPlainText(text)
     -- Trim spaces and new lines at start and end
     text = text:gsub("^[\n%s]*", "")
     text = text:gsub("[\n%s]*$", "")
+    -- Trim non-breaking spaces from the start
+    text = text:gsub("^\xC2\xA0\xC2\xA0\xC2\xA0\xC2\xA0", "")
     return text
 end