Polish desktop navigation chrome

[klopez4212]

Summary

• Add an in-flow sidebar-colored navigation bar for the sidebar, back, and forward controls.
• Round and faintly stroke the main content panel so it tucks cleanly under the new chrome.
• Normalize header/search spacing around 36px content rows and restore list-view top padding.

Snapshots

Channel chrome

Agents panel

Validation

• pnpm --dir desktop exec biome check ...
• pnpm --dir desktop run --if-present typecheck
• pnpm --dir desktop run --if-present check:px-text
• git diff --check
• just desktop-screenshot for channel chrome and agents panel snapshots
• code-checks: reviewed changed lines; fixed the only discovered padding consistency issue

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