Mixed-content copy bypasses table sanitizer, preserves dark theme, and blanks headers

Bug description

The fix for #4945 sanitizes an isolated full-table selection, but copying a normal assistant response containing mixed rendered content still falls back to native browser clipboard serialization.

When the selected range contains headings and one or more rendered Markdown tables, pasting into a web-based rich-text editor carries over Hermes WebUI's dark-theme presentation styling. The tables retain their rows and values, but each column-header row pastes as blank cells. This makes otherwise valid quote/report output unsuitable for direct use in external systems.

This remaining case was reported in a follow-up comment on #4945, but the closed issue was not reopened.

Steps to reproduce

1. Generate an assistant response containing a heading and multiple Markdown tables, for example:

   ## Monthly Summary
   
   ### Example Site A
   
   | Product Name | Qty | Unit Cost | Total |
   | --- | --- | --- | --- |
   | Example Service A | 10 | $1.00 | $10.00 |
   | **Monthly Subtotal** |  |  | **$10.00** |
   
   ### Example Site B
   
   | Product Name | Qty | Unit Cost | Total |
   | --- | --- | --- | --- |
   | Example Service B | 20 | $2.00 | $40.00 |
   | **Monthly Subtotal** |  |  | **$40.00** |
   

2. Select the complete rendered response, including the headings and both tables.

3. Copy it from Hermes WebUI.

4. Paste it into a web-based rich-text editor.

Actual behavior

• Hermes WebUI dark backgrounds, text colors, borders, and spacing are preserved in the pasted content.
• Table body rows and values are present.
• The column-header row is rendered as empty cells. Labels such as Product Name, Qty, Unit Cost, and Total are missing.
• The result differs from copying one complete table by itself because the mixed selection bypasses the table-only clipboard sanitizer.

Expected behavior

Copying a mixed rendered response should place clean, theme-neutral semantic HTML on the clipboard while preserving:

• headings and paragraphs;
• lists and emphasis;
• complete table structure;
• visible <th> header text;
• bold subtotal/total cells.

Hermes-specific classes, dark-theme colors/backgrounds, borders, fonts, spacing, and interactive/sortable-header controls should not be included in clipboard HTML.

A plain-text clipboard representation should also remain available as a readable Markdown or text fallback.

Regression / scope note

#4945 was closed by the full-table-only sanitizer released in v0.51.684. The implementation deliberately keeps native copy behavior for prose and mixed prose/table selections. That leaves the common workflow of selecting an entire assistant response unresolved.

This is therefore not a report that isolated full-table copying is still broken. It is the remaining mixed-content selection case exposed by the limited scope of the original fix.

Environment

• App: Hermes WebUI
• Surface: browser-based WebUI
• Source theme: dark
• Destination: web-based rich-text editor
• OS: Windows
• Reproduced after the #4945 fix was released

Suggested acceptance coverage

Add clipboard regression tests for selections that contain:

1. heading + one complete table;
2. heading + multiple complete tables;
3. paragraph/list + table;
4. mixed content before and after a table.

For each case, assert that text/html is theme-neutral, table headers remain visible semantic header cells, and text/plain remains readable.