refactor(ui): move model selection to Astryx Selector

[Astro-Han]

Summary

Refs #1565 (Slice 8, flat PR B: model selection/search).

• Replace the Maka-owned model picker combobox, Popover, query/filter, highlight, scroll, keyboard, focus, listbox, and option authorities with a product composition over Astryx Selector 0.1.9.
• Keep product ownership limited to provider/model shaping, provider sections and option marks, known or unknown current-value presentation, and persistence actions.
• Move Thinking Level out of the model popup footer into an adjacent, explicit Astryx Selector while preserving new-chat and active-session persistence.
• Make AppShell the single session-keyed action owner for both model and Thinking Level mutations. The adjacent controls share one pending state, exclude same-session races in both directions, and leave another session usable while a prior session settles.
• Use public Selector values as the provider-mark lookup seam instead of relying on Astryx to preserve product-only option fields.
• Update the composer, chat model switcher, General Settings default-model control, related CSS, copy, contracts, Storybook states, and Electron journeys.
• Delete the now-unused legacy picker trigger recipe from ui.tsx; no Collapsible, Accordion, Toggle, or Progress implementation is changed.

This is an independent flat boundary: it builds and tests from current main, does not depend on another PR, and does not change CommandPalette, SearchModal, mention popup, PowerSearch, chat turns, tool activity, or task ledger ownership.

Verification

• npm --workspace @maka/ui test — 285 passed.
• npm --workspace @maka/desktop test — 2700 passed, including workspace dependency builds and console/a11y/copy checks.
• npm --workspace @maka/desktop run build — passed.
• npm --workspace @maka/desktop run typecheck — passed.
• npm --workspace @maka/desktop run build-storybook — passed.
• npm run lint — passed.
• npm run format:check — passed.
• Targeted Electron Playwright — 5 passed: Settings provider-mark containment/alignment; empty active-descendant behavior; keyboard scrolling; popup hit testing/focus restoration/long-label alignment; and Thinking Level create/reload persistence plus a real active-session model switch.
• Behavior tests cover same-session model/Thinking exclusion in both directions, cross-session availability during a delayed mutation, failure release/retry, provider grouping/value-to-mark mapping, and unknown/empty current values.
• Storybook covers default, search, no results, unknown current, empty catalog, pending, adjacent Thinking Level, and the real General Settings empty-model-catalog state.
• Captured and inspected a real Electron model-selector frame at apps/desktop/test-results/model-selector-live.png (local ignored evidence, not a screenshot baseline). The live DOM/computed-style checks cover trigger/popup hit testing, provider mark/label alignment, long labels, and adjacent Thinking Level alignment.

Capability changes

• The former pinned-exempt search behavior is intentionally removed. Unknown-current and not-set values are ordinary Astryx options and are filtered like every other option.
• Astryx 0.1.9 searches option labels, so matching only a provider section heading is no longer supported.
• Provider marks remain on model options. The trigger-only mark is removed because Astryx optimistically changes its selected label while the controlled product value is still persisting; keeping a separately controlled mark could display a new model label with the old provider identity.
• Astryx owns its no-result presentation and search accessible name; Maka no longer carries parallel localized empty/search-ARIA copy. The product-owned search placeholder remains localized.

Review focus

Review the ownership subtraction: Maka maps product data and owns session-keyed persistence admission; Astryx remains the sole search, option, optimistic UI, keyboard, focus, scrolling, and popup authority. General Settings keeps its existing settings-save owner, and new-chat selection is synchronous local draft state.