Persist last selected save per project

[liurenfeng94-ops]

Part of #18

• add a project settings file in the extension project storage directory
• persist the last selected save name for each project
• restore that save when reopening a project if it still exists
• cover project settings parsing and persistence with tests

Validation:

• pnpm vitest run test/project-settings.test.ts
• pnpm typecheck
• pnpm exec eslint .
• pnpm build