refactor(ui-react): consolidate app shell session state

[Astro-Han]

Summary

• Add a local AppShell session UI state controller for the per-session UI maps.
• Move AppShell to the controller-backed hook while preserving same-frame refs for streaming and session-event health handlers.
• Fix nested map updates so thinking_delta / thinking_complete can update thinking text and truncated flags atomically from the caller's point of view.
• Update source contracts so they protect retry/model/permission/cleanup behavior without pinning direct useState placement.

Why

Refs #477

AppShell kept 11 per-session UI maps as separate top-level state entries. Every new per-session UI concern had to touch the AppShell top level, and archive/delete cleanup had to remember every individual map. This PR gives those maps one local owner without introducing an external store.

Scope

Changed:

• app-shell-session-ui-state.ts owns the AppShell per-session UI controller, setters, same-frame refs, and centralized session cleanup.
• app-shell.tsx reads/writes those maps through useAppShellSessionUiState().
• Session retry, row action cleanup, permission mode, and sticky model contracts now assert the controller-backed behavior instead of direct useState placement.
• The controller uses a single map-key list plus generic map update/clear helpers, avoiding the previous 11-case action switch and duplicated cleanup list.
• A regression test covers the nested thinking updater shape: a truncated thinking_delta sets the flag, and a short thinking_complete replaces text and clears the flag.

Not included:

• No visual redesign.
• No external state store.
• No targeted memo work.
• No effect dependency rewrite beyond preserving the existing ref-stability behavior.

Verification

• GREEN: npm run -w @maka/desktop test (1847 passed)
• GREEN: npm run -w @maka/desktop build:renderer (passes; existing Vite large chunk warning remains)

User-facing impact

None expected. This is a React state-ownership refactor; UI copy, layout, and behavior should remain unchanged.

Reviewer notes

The PR is split into rollback-sized commits:

• refactor(ui-react): add app shell session ui reducer
• refactor(ui-react): use reducer for app shell session state
• fix(ui-react): preserve nested session ui updates