Refactor web app orchestration and import pipeline

[dills122]

Summary

• Splits the web app shell into focused state, refresh, export, and theme helpers.
• Extracts repository import behavior out of the browser Worker message handler.

What Changed

• Reduced ForageApp.svelte by moving app state/display helpers into app-state.ts.
• Added app-refresh.ts to assemble local IndexedDB and Worker session/config state into UI state patches.
• Added export-data.ts and theme.ts for export/download and theme behavior.
• Added import-pipeline.ts so the import Worker delegates repository import orchestration.
• Added import-messages.ts for import progress and terminal message formatting.
• Added focused tests for the extracted app state, refresh, and import message helpers.

Validation

• pnpm --filter @forage/web test
• pnpm --filter @forage/web typecheck
• pnpm --filter @forage/web build
• pnpm lint
• pnpm check

Scope Notes

• This is behavior-preserving cleanup only.
• The Worker auth/server god-file split is intentionally left for a separate security-sensitive refactor branch.