(feat) redesign install section with OS tab switcher

[alexey-pelykh]

Summary

• Move install UI into the hero section — no more separate "Get started" block at the bottom
• Add pill tab switcher: macOS / Linux | Windows | npm
• Use remoteclaw.sh as the primary install URL (was remoteclaw.org/install.sh)
• Auto-detect visitor OS to pre-select the right tab (Windows detection)
• Each tab shows the primary command with copy-to-clipboard
• Remove misleading "Install without root" expandable (the default install doesn't need root on macOS)

Test plan

• Verify macOS/Linux tab shows curl -fsSL remoteclaw.sh | sh with copy button
• Verify Windows tab shows irm remoteclaw.org/install.ps1 | iex with copy button
• Verify npm tab shows npm install -g remoteclaw with copy button
• Verify tab switching works and preserves selected state
• Verify copy buttons work (clipboard + "copied!" feedback)
• Verify responsive layout on mobile
• Verify "Read the docs" link still works

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