🎨 Palette: Add tooltip to transaction cost input

[aarjava]

Added a help tooltip to the 'Transaction Cost (bps)' input in the sidebar to clarify the meaning of 'bps' (basis points) for users. This follows Palette's philosophy of adding helpful context to improve accessibility and understanding of financial jargon.

Changes:

• Added help parameter to st.number_input for transaction cost in src/dashboard.py.
• Formatted src/dashboard.py with Black and Ruff (sorted imports).
• Added entry to UX journal .Jules/palette.md.

Verification:

• Verified visually using Playwright (screenshot captured).
• Ran existing tests (pytest tests/), all passed.

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PR created automatically by Jules for task 17413612790298182360 started by @aarjava

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