Improve split pane behavior and UI polish

[Finesssee]

Summary

This pulls the split-pane work out into its own PR instead of burying it inside the larger Pi/Harness parity branch.

It does three things:

• prevents split panes from collapsing into unusable slivers and auto-repairs bad restored ratios
• changes generic split actions to create a new empty pane instead of duplicating the active tab
• polishes split-pane UI so empty panes, active focus, resize handles, and narrow layouts feel intentional

What changed

Split sizing and recovery

• raise the minimum pane threshold to 20%
• normalize collapsed split pairs to 40/60 on drag release
• repair previously bad split ratios on restore/startup

Split semantics

• generic split actions now open an empty pane and focus it
• tab-context split actions still open the selected tab in the new pane
• generic UI copy now says Split Pane

Split UI polish

• always show structural tab chrome for empty panes
• make active pane focus clearer with calmer hierarchy
• slim the resize handle visually while keeping a generous hit area
• hide nonessential tab chrome in narrow panes so cramped splits stay usable

Verification

Ran on this branch:

• bun typecheck
• bun lint
• bunx vp test run src/features/panes/stores/pane-store.test.ts src/features/panes/utils/pane-tree.test.ts
• bunx vp build
• git diff --check

Watched-app verification used the existing local X11/VNC flow and screenshots:

• /tmp/athas-split-empty-after-key.png
• /tmp/athas-empty-split-normal.png
• /tmp/athas-empty-split-narrow.png
• /tmp/athas-tab-context-split.png

Notes

The repo's full pre-push gate on this branch still trips over upstream master Rust issues that are unrelated to this PR:

• cargo fmt --check --all reports formatting drift in crates/runtime and crates/tooling
• cargo check --workspace fails on Linux in src-tauri/src/commands/ui/window.rs because hidden_title(...) is not available on this target

I left those out of this PR on purpose so the split-pane extraction stays scoped.