Port upstream 0.55.0: scale single-quota tray icons

[Finesssee]

Summary

• port upstream CodexBar v0.55.0 single-quota icon scaling from Fix single-quota menu bar icon scaling steipete/CodexBar#3155
• when a provider has only one meaningful quota, render it as one full-width tray meter instead of pairing it with an unavailable/duplicated second lane
• cover the case where the sole meaningful quota arrives in the secondary slot

Upstream evidence

• target release: v0.55.0
• upstream commit: daa16fb84 / Fix single-quota menu bar icon scaling steipete/CodexBar#3155

Validation

• cargo test --manifest-path apps/desktop-tauri/src-tauri/Cargo.toml selected_tray_percent --no-fail-fast -> 4 passed
• cargo test --manifest-path apps/desktop-tauri/src-tauri/Cargo.toml meaningful --no-fail-fast -> 2 passed
• focused rustfmt and git diff --check -> pass

Scope

Isolated Windows tray presentation port. Fresh Windows CUA proof remains part of the final v0.55 UI closure gate.

[Finesssee]

Thermo-nuclear re-review

Previous finding is closed: two-lane tray selection now lives beside canonical metric selection and chooses a distinct companion lane instead of blindly re-emitting secondary. The informational-primary + secondary + tertiary duplicate regression passes, as do strict desktop Clippy checks.

Verdict: APPROVED.