Add new session icon to sessions sidebar header

[codenomadbot]

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[shantur]

@CodeNomadBot

Check the diff of this PR, we are doing
Also align the new session and searchn icon buttons size and styles to match the pin and info icon buttons

[codenomadbot]

Updated the PR diff:

• Aligned the “New session” (PlusSquare) and “Search” icons to the same visual size as the existing pin/info icon buttons by switching both Lucide icons to w-5 h-5 (20px) in packages/ui/src/components/instance/shell/SessionSidebar.tsx.
• Removed the inactive-state opacity tweak on the search icon so it matches the pin/info button weight; the active state is still indicated via the existing sx background/color.

Pushed as fix(ui): align sidebar header icon sizes (commit 6ed1e09) onto the PR head branch.

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