chore(OUK): Upgrade version, fix migration issues

[misi-bp]

Description

Upgrades the lungo frontend from Open UI Kit (OUK) 1.6 to 2.1 and updates call sites for OUK 2’s breaking API changes, while keeping MUI at ^7.0.1 so the app stays on the same Material UI major/minor line that OUK 2 declares and bundles against. We intentionally did not upgrade MUI to v8/v9: OUK 2.1.0 depends on @mui/material ^7.0.1, and aligning on that version avoids duplicate MUI installs, conflicting Theme types, and compat shims (overrides, icon aliases, palette augmentations) that a higher MUI version would require.

Dependency changes

• @open-ui-kit/core: ^1.6.0 → ^2.1.0
• @mui/material / @mui/icons-material: remain ^7.0.1 (resolved to 7.3.x; deduped with OUK)
• @testing-library/dom: ^10.4.1 added to devDependencies. Explicitly listing @testing-library/dom documents and guarantees the peer dependency that @testing-library/react v16 needs at test time, so fresh npm install runs stay reliable instead of depending on implicit hoisting or peer-resolution quirks.

OUK 2 migration

• Theme mode: OUK 2 replaces isDarkMode / defaultDarkMode with ThemeMode + mode / setMode. Added useAppThemeMode as a thin wrapper for boolean helpers (isDarkMode, setIsDarkMode, toggleTheme) used across navigation, chat, and persistence.
• Modals: Modal / ModalContent / ModalTitle → Dialog / DialogContent / DialogTitle in badge, policy, OASF, and info modals.
• Dropdown options: OUK no longer exports DropdownOption; added src/types/dropdownOption.ts for suggested-prompts menu typing.
• ListItemButton: Removed invalid type="button" props where OUK 2’s typings no longer accept them.

Naming and structure

• Renamed project-specific theme files/symbols to neutral names (CompactTheme, compactThemeOptions, useAppThemeMode, app-shell panel id, theme-mode storage key with legacy read fallback).
• Split railBackgroundImage into transportRailBackground.ts so TransportRail.tsx only exports a component (fixes react-refresh/only-export-components / Fast Refresh lint).

Verification

• npm run check (lint, format, typecheck, 133 tests) passes
• npm run build passes

Issue Link

#684

Type of Change

• Bugfix
• New Feature
• Breaking Change
• Refactor
• Documentation
• Other (please describe)

Checklist

• I have read the contributing guidelines
• Existing issues have been referenced (where applicable)
• I have verified this change is not present in other open pull requests
• Functionality is documented
• All code style checks pass
• New code contribution is covered by automated tests
• All new and existing tests pass