refactor(web): formalize chat state budgets

[BetterAndBetterII]

Summary

• extract small helper modules for workspace browser session, file editor selected actions, and terminal panel state so the chat state stores stay maintainable
• shrink the remaining oversized chat state modules below the formal featureStateModule threshold and drop the temporary 350-line bypass
• update the shared file-budget config and README so the enforced lib/features/**/*.svelte.{ts,js} hard limit is now 325

Validation

• PATH=$HOME/.nvm/versions/node/v22.22.1/bin:$PATH pnpm --dir web run ci
• PATH=$HOME/.nvm/versions/node/v22.22.1/bin:$PATH pnpm --dir web exec vitest run src/lib/features/chat/terminal-manager.test.ts src/lib/features/chat/project-conversation-workspace-browser-state.test.ts src/lib/features/chat/project-conversation-workspace-browser-git-state.test.ts src/lib/features/chat/project-conversation-workspace-browser-git-checkout.test.ts src/lib/features/chat/project-conversation-workspace-browser-git-checkout-refresh.test.ts
• GitHub Actions PR checks: Frontend Checks, Frontend API Audit Check, and Desktop Checks all passed on PR refactor(web): formalize chat state budgets #772

Closes ASE-330.