refactor: split frontend god object

[stackbilt-admin]

Context

The frontend application is currently implemented as one 905-line file: frontend/app.js. This violates the architecture constraint in ARCHITECTURE.md that source modules stay under 400 lines.

The file owns too many responsibilities at once:

• config and API auth helpers (frontend/app.js:8)
• inline icon registry (frontend/app.js:30)
• global app state (frontend/app.js:41)
• routing and page dispatch (frontend/app.js:52, frontend/app.js:64)
• search rendering and events (frontend/app.js:94)
• chat rendering, events, and API calls (frontend/app.js:202, frontend/app.js:310)
• upload rendering, multipart upload, and ingestion polling (frontend/app.js:417, frontend/app.js:562, frontend/app.js:633)
• settings and health checks (frontend/app.js:657, frontend/app.js:713)
• assistant markdown/think formatting (frontend/app.js:751)
• utility formatting, toasts, and bootstrapping (frontend/app.js:808, frontend/app.js:858, frontend/app.js:884)

Recommendation

Split frontend/app.js into small responsibility-focused modules while preserving the existing static frontend approach.

Suggested first pass:

• frontend/js/api.js for API base URL, API key access, and fetch helpers
• frontend/js/state.js for app state and navigation/render invalidation
• frontend/js/icons.js for inline icon templates
• frontend/js/pages/search.js
• frontend/js/pages/chat.js
• frontend/js/pages/upload.js
• frontend/js/pages/settings.js
• frontend/js/pages/detail.js
• frontend/js/formatting.js for markdown, message parsing, date/byte formatting, and escaping
• frontend/js/toast.js for toast rendering/lifecycle
• keep frontend/app.js as a thin bootstrap and page dispatcher

Acceptance criteria

• No frontend JS module exceeds 400 lines.
• Behavior of search, chat, upload, settings, and detail views is unchanged.
• Existing static deployment remains simple, with no required bundler unless deliberately introduced.
• Shared helpers are imported rather than duplicated.
• Add at least light coverage or a manual verification checklist for the major UI flows.

Verification baseline

Current baseline from the audit:

• npm test passes: 12 tests across 2 files.
• npm run type-check passes.