refactor(ui,settings): Round D — SettingsSegmented primitive + retire 4 hand-rolled radiogroup helpers

[jackwener]

Summary

@WAWQAQ msg `f1461d30` 「用库的应该用库」: continue collapsing the hand-rolled wrappers in SettingsModal.tsx into shared @maka/ui primitives.

Primitive

packages/ui/src/primitives/settings-segmented.tsx wraps Base UI's ToggleGroup (single-select by default). Caller's className (legacy .settingsSegmented) still owns the visual chrome — the primitive only contributes Base UI's behavior contract.

Migration

• function Segmented deleted from SettingsModal. 3 call sites use the imported primitive aliased as Segmented so their JSX is byte-identical.
• .settingsSegmented button[data-active=\"true\"] → [data-pressed] (Base UI's Toggle state attribute).

Helper retirement

With Theme/Palette migrated via #263 (ChoiceCard) and Segmented migrated here, all four hand-rolled keyboard-nav helpers in SettingsModal are dead:

• onSettingsRadioGroupKeyDown, focusRadioValue, radioTabIndex — deleted.
• nextRadioId import — deleted from SettingsModal.

nextRadioId itself stays in model-table-keyboard.ts because ProvidersPanel's model default picker keeps its hand-rolled radiogroup.

Contract test pivot

Keyboard-nav assertion in settings-theme-contract.test.ts flips: was 'Segmented uses the legacy helpers' → 'Segmented imports SettingsSegmented; the local function Segmented declaration is gone; the 4 hand-rolled helpers are gone'.

Test plan

• pnpm -F @maka/ui build — clean.
• tsc --noEmit -p apps/desktop/tsconfig.renderer.json — only the pre-existing unrelated TS2366.
• settings-theme-contract.test.ts — 6/6 pass.
• renderer-style-pruning-contract.test.ts — 2/2 pass.
• Visual smoke: Settings → 模型 / 通用 pages with Segmented pills still toggle correctly; arrow keys cycle between options; active pill has the white background + light shadow chrome.