fix(chat): follow-ups from the list-indent review (#2096) #2108

Follow-up to #2096 (LOW notes from the merge pass's review; none blocked).

• Scientific-notation starts: 9000000000000000000000. first renders start="9e+21", which the browser reads as 9, while the gutter is sized from the parsed start; 10. can under-indent (also on base). Apply the normalized start to the emitted start attribute too (src/components/Markdown.tsx ~230).
• Indent applies wherever Markdown renders: .markdown-list is global, so release notes and file previews get the 3ch indent too (harmless; worth a line in the PR's notes).
• Streaming shift: a list that grows from 9 to 10 items moves every earlier item right by one digit.
• RTL: the indent is still padding-left; padding-inline-start would follow dir.
• CJK bullets: 2ch assumes a one-character bullet; HackGen-style fonts may draw • double width.