refactor(web): split app.js into per-view modules + shared UI helpers

[smochan]

Summary

Track A2, stacked on #69. Pure extraction, zero behavior change:

• app.js: 919 → 240 lines — keeps only data fetch, render dispatcher, sidebar/theme shell
• 6 new view modules (views/overview|hld|flows|matrix|sankey|treemap.js) exposed via window.CGViews.*
• ui/helpers.js: shared esc/format/tooltip/toast/mermaid-theme/kind helpers via window.CGUI.* with per-view destructuring (matches the repo's no-bundler script-tag architecture)
• index.html loads helpers → views → app.js in dependency order; no-CDN guard test extended to the new files

Implemented by a Sonnet subagent against a Fable spec; reviewed (namespace wiring, window.state exposure, node --check on every file, payload smoke test) by the orchestrating session.

Test plan

• Full suite 708 passed; ruff + mypy clean
• node --check on all new/changed JS
• Dashboard payload smoke test on a fresh tiny repo

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