docs: add UI enhancement phases and consolidated backlog

[rad1092]

Motivation

• Provide a clear, actionable plan for the next UI implementation turns so work can start immediately.
• Consolidate remaining improvement items (usability, parsing, performance, format support) in a single reference for prioritization and tracking.

Description

• Add NEXT_STEPS_UI_AND_BACKLOG.md which defines Phase 1–5 interface work (simplification, natural-language requests, state/feedback, dashboard, async charts) and a prioritized backlog of analysis, code, and file-format improvements.
• Include execution order and verification checkpoints so the next turn can pick up implementation quickly.

Testing

• Ran pytest -q and all tests passed (26 passed).

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve decimal-comma values when normalizing numbers

Removing every comma in _to_float silently corrupts locale-formatted decimals: inputs like "1,5" become 15.0 and "12,5%" become 125.0, so summarize_reader will compute incorrect numeric stats and dtype classification instead of leaving these cells non-numeric. This regression is introduced by the new normalization path and affects any CSV that uses comma as the decimal separator.

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