fix: add tabs instead of automatically splitting a fourth tile

[16francej]

New sessions add a tab to the selected pane once three tiles are open, while explicit pane splitting remains available.

• Verification: 960 web tests passed; web typechecks and changed-file lint passed.
• Browser: reproduced before fix; verified selected-pane targeting, repeated tabs, explicit fourth-tile splitting, and four-tile fallback afterward.

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