v0.9.0 UX: model picker shows awkward single-row range titles

[Hmbown]

Summary

When a scrollable picker pane has exactly one visible row, the model picker title still renders a range like Model 2-2/3. That is technically accurate but reads awkwardly and looks like a formatting bug; it should read like a single position, e.g. Model 2/3.

Evidence

Isolated installed-build capture:

/tmp/codewhale-uiux.Fc1Zdl/artifacts/tui/model80.screen.txt:9
┌ Model 2-2/3 ─────────────────────────────────────────────────────────────┐

The Thinking pane in the same capture has multiple visible rows, so Thinking 3-4/4 is fine. The bug is the single-visible-row case.

Source pointer:

• crates/tui/src/tui/model_picker.rs: render_pane formats start + 1 through end whenever the pane is scrollable.

Acceptance

• Single-visible-row panes render as {title} N/total, not {title} N-N/total.
• Multi-row panes keep a real range like {title} 3-4/4.
• Add/update a model picker render/unit test for the one-row visible window case.

Notes

Visual polish only; model selection behavior is not implicated.

[nightt5879]

Opened PR #4203 for this issue: #4203. CI and Claude review are green; no review threads were opened. I do not have permission from this account to flip labels off agent-ready, so leaving the issue open for the PR merge to close via Fixes #3995.