fix: mermaid duplicated ids + gantt default size

[pqoqubbw]

Summary

Test Plan

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Note

Low Risk
Localized UI rendering/config changes; main risk is the regex-based SVG rewrite missing/over-matching marker IDs and altering diagram rendering in edge cases.

Overview
Fixes Mermaid rendering issues when multiple diagrams are present by post-processing the generated SVG to prefix known marker ids and their url(#...) references, preventing cross-diagram SVG marker collisions.

Also sets a default Gantt useWidth (800) in the Mermaid config, and adds a Storybook DuplicateDiagrams tabbed story to exercise the duplicate-marker scenario.

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