[feat] Add explicit client-chrome language preference

[ThreeLight2000]

Motivation

Chrome may be configured in English while the user prefers Context Shelf in Korean. The current i18n implementation follows Chrome language automatically and has no user override.

Scope

• Add Auto / 한국어 / English selection in client-chrome connection settings.
• Persist the preference separately from AppData under contextShelf:locale-preference:v1.
• Apply changes immediately and synchronize open extension surfaces.
• Keep Auto as the default and preserve locale-aware date and clipboard output.

Acceptance criteria

• Explicit Korean preference works independently of Chrome language.
• Explicit English preference works independently of Chrome language.
• Auto follows the current Chrome/browser locale.
• Invalid or missing stored values fall back to Auto.
• Preference persists across extension restarts and surfaces.
• Existing typecheck, tests, and production build pass.

Verification plan

• Add unit tests for preference resolution and storage fallback.
• Verify immediate UI switching and persistence across popup, side panel, and options.
• Run the client-chrome typecheck, test, and build commands.

[ThreeLight2000]

Implementation started on branch codex/1-client-chrome-language-selector. Existing client-chrome i18n groundwork is being extended with an explicit Auto / 한국어 / English preference, separate storage, immediate switching, and cross-surface synchronization.

[ThreeLight2000]

Implementation and verification complete in PR #2: #2

• Added Auto / 한국어 / English preference in client-chrome connection settings.
• Persisted preference under contextShelf:locale-preference:v1 without changing AppData.
• Added immediate locale switching and cross-surface synchronization.
• pnpm --filter context-shelf verify passed.
• pnpm verify passed.

Issue remains open until the PR is merged.