feat: improve markdown rendering in Expert panel

[andypalmi]

Summary

• Add scoped heading styles (h1–h6) for Expert chat messages — compact sizes suitable for the sidebar panel
• Add table wrapper with copy-to-clipboard button (tab-separated for spreadsheet paste), cell borders, and header styling
• Add blockquote styling with accent left border
• Add ordered list styling (was unstyled, numbers were missing)
• Add GFM task list / checkbox rendering with proper checked/unchecked visual distinction
• Move code block copy button to the left for consistency

All styles are scoped to StreamableContent.vue — no impact on other FlowFuse pages.

Test plan

• Verify headings h1–h6 render with visible size hierarchy in Expert chat
• Verify tables have borders, header background, and working copy button
• Verify blockquotes show accent left border and subtle text
• Verify ordered lists show numbers
• Verify unordered lists still show bullet points
• Verify GFM checkboxes render checked vs unchecked distinctly
• Verify code block copy button is on the left (with and without language label)
• Verify no style regressions on other FlowFuse pages

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.84%. Comparing base (c4552d6) to head (f9629e0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7512   +/-   ##
=======================================
  Coverage   76.84%   76.84%           
=======================================
  Files         410      410           
  Lines       20922    20922           
  Branches     5093     5093           
=======================================
  Hits        16078    16078           
  Misses       4844     4844           

Flag	Coverage Δ
backend	76.84% <ø> (ø)

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