feat(frontend): rename navigation and add page instructions for UX

[charlie83Gs]

Summary

• Rename Syntheses → Investigate (/investigate) and Research → Grow Graph (/grow-graph) to better communicate purpose to new users
• Split sidebar navigation into Workflow (Home, Investigate, Grow Graph) and Data (Nodes, Edges, Facts, Sources, Seeds, Candidates) groups with a visual divider
• Add instructional subtitles on every page to guide new users through the system
• Restructure home page with updated card descriptions and a suggested workflow flow
• Add MCP mention to API Tokens page subtitle

Test plan

• pnpm lint passes
• pnpm type-check passes
• pnpm test — all 123 tests pass
• Verify sidebar labels "Investigate" and "Grow Graph" render in expanded and collapsed (tooltip) states
• Verify divider renders between "Grow Graph" and "Nodes"
• Verify /investigate and /grow-graph routes load correctly
• Verify all home page card links navigate to correct pages
• Verify back links on investigate detail page work
• No broken links remain to old /syntheses or /research paths

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