🎨 Palette: Enforce plain text in QTextEdit

[dieterolson]

💡 What:
Added .setAcceptRichText(False) to all four QTextEdit instances in the application (chat_panel.py, hub_dialog.py, settings_panel.py, and setup_dialog.py).

🎯 Why:
By default, QTextEdit accepts rich text (HTML) when pasting. If a user pastes text containing formatting into a field like "System prompt" or the application logs, it renders the HTML formatting instead of treating it as plain text. This can cause UI inconsistencies and redressing issues.

📸 Before/After:
Before: Pasting formatted text or HTML into _system_prompt rendered it as rich text, causing font mismatches and potential UI redressing.
After: Pasting formatted text or HTML into _system_prompt pastes it as raw, plain text, keeping the application's intended style.

♿ Accessibility:
Improves visual consistency and predictability for users relying on keyboard copy-pasting, preventing accidental rendering of confusing or unreadable formatting in configuration fields.

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PR created automatically by Jules for task 4420950828022961881 started by @dieterolson

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[github-actions]

⚠️ SPEC.md Update Required

Source files were modified in this PR but SPEC.md was not updated.

Per the Specification-Driven Development Policy:

• Any PR that adds, removes, or changes functionality must update SPEC.md
• Update the relevant sections (features, architecture, tests, dependencies, etc.)
• Bump the Spec Version if making substantive changes

To resolve:

1. Update SPEC.md to reflect your changes, OR
2. Add the spec-exempt label if this PR genuinely doesn't affect the spec (e.g., pure refactor with no behavior change)

This check is enforced by the spec-check CI workflow.

[dieterolson]

Closing in favor of consolidated batch PR #227.