refactor(web): tighten feature state budgets

[BetterAndBetterII]

Summary

• extract reusable workspace file editor runtime helpers so autosave and working-set logic live outside the largest state store
• collapse repeated selected-editor mutations in the workspace file editor store and trim browser / terminal state helpers
• tighten the lib/features/**/*.svelte.{ts,js} hard file budget from 350 to 325 now that the remaining state modules fit again

Validation

• PATH=$HOME/.nvm/versions/node/v22.22.1/bin:$PATH pnpm run lint:structure
• PATH=$HOME/.nvm/versions/node/v22.22.1/bin:$PATH pnpm run lint
• PATH=$HOME/.nvm/versions/node/v22.22.1/bin:$PATH pnpm run check
• PATH=$HOME/.nvm/versions/node/v22.22.1/bin:$PATH pnpm run lint:deps
• PATH=$HOME/.nvm/versions/node/v22.22.1/bin:$PATH pnpm exec vitest run 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 src/lib/features/chat/terminal-manager.test.ts
• PATH=$HOME/.nvm/versions/node/v22.22.1/bin:$PATH pnpm run build

Ticket

• ASE-341