fix(modals): increase sm/md/lg modal widths

[davideweaver]

Summary

• silo-modal-sm: 300px → 400px
• silo-modal-md: 400px → 560px
• silo-modal-lg: 560px → 800px
• Accompanying git-explorer layout and CSS cleanup

Test plan

• Open a small modal (e.g. confirmation dialog) — verify it renders at 400px width without feeling too wide
• Open a medium modal (e.g. branch manager) — verify 560px fits content comfortably
• Open a large modal (e.g. extensions page) — verify 800px gives adequate breathing room
• Check both dark and light themes

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