feat: non-technical web UI — upload → brain → ask

[rajnavakoti]

Description

Simplified web interface for non-technical users. Three steps: upload your documents, see the extracted brain, ask questions and see gaps. No terminal, no YAML, no config files. The viewer already has all the pieces — this is a guided flow that chains them together.

Acceptance Criteria

• Landing page with clear 3-step flow: Upload → Explore → Ask
• Step 1: drag-and-drop upload (reuse upload component from #143)
• Step 2: auto-redirect to Digest page showing the extracted brain
• Step 3: Ask page for questions, with gap highlights in plain language
• Ontology auto-selected (LLM-generated from seed context or default)
• No CLI commands, no config files, no terminal required
• Mobile-responsive at 320px

Out of Scope

• User accounts / multi-tenant
• Custom ontology editing in UI
• Depends on #143 (web upload), #133 (LLM-generated ontology)