perf(web): reduce startup bundle pressure for optional runtimes

[limityan]

Summary

This PR contains the completed stage 0-4 Web UI startup bundle optimization work, squashed into two commits and without the design document.

It keeps chunkSizeWarningLimit unchanged and focuses on removing accidental startup dependencies rather than hiding the warning.

Optimization Plan

• Add repeatable bundle measurement guardrails:
  • scripts/report-web-bundle-size.cjs reports raw and gzip sizes from dist/assets.
  • scripts/report-web-main-static-graph.cjs reports the main.tsx static graph and can assert that optional runtimes stay unreachable from startup.
• Narrow startup tool initialization:
  • main.tsx now imports ./tools/initializeTools instead of the full ./tools barrel.
  • Git and LSP initialization moved behind narrow initializer modules while keeping existing barrels compatible.
• Replace dynamic Lucide namespace lookups:
  • Context menu, MiniApp icons, and smart recommendations now use explicit icon maps.
  • Existing fallback behavior is preserved for unknown icon names.
• Defer optional Markdown runtimes:
  • Mermaid loads through a cached dynamic import in MermaidService only when Mermaid content needs rendering, validation, or export.
  • Markdown code blocks and CodePreview use a shared async Prism loader.
  • Code content remains readable before Prism finishes loading, so normal chat and plain Markdown rendering do not wait for syntax highlighting.

Optimization Results

Measured from the stage 0 baseline to the final stage 4 build:

• Largest entry JS: 9953.0 KiB raw / 2802.0 KiB gzip -> 8242.6 KiB raw / 2286.1 KiB gzip.
• Entry JS reduction: -1710.4 KiB raw / -515.9 KiB gzip.
• main.tsx static graph: 1067 -> 1013 local modules.
• Total JS/CSS raw size: 14332.4 KiB -> 13952.4 KiB.
• Total JS/CSS gzip size: 3905.1 KiB -> 3930.0 KiB.
  • The small gzip increase is an intentional experience tradeoff from async Prism language assets: code blocks stay readable first, then enhance, while normal startup no longer prepays the syntax highlighter runtime.
• Static graph assertions confirm startup no longer reaches mermaid or react-syntax-highlighter, and main.tsx no longer imports ./tools directly.
• dist/index.html does not preload mermaid, prism, or react-syntax assets.

Experience Protection

• Did not lazy-load the whole SessionScene or the whole Markdown renderer.
• Git/LSP startup remains after-render initialization through narrower imports.
• Mermaid and code highlighting are optional enhancements, not blockers for plain chat or plain Markdown.
• Unknown icon names keep fallback UI behavior instead of rendering blank controls.

Verification

• pnpm run lint:web
• pnpm run type-check:web
• pnpm --dir src/web-ui run test:run
• pnpm --dir src/web-ui run build
• pnpm run verify:monaco-assets
• node scripts/report-web-bundle-size.cjs --top=20
• node scripts/report-web-main-static-graph.cjs --assert-external-unreachable=react-syntax-highlighter --assert-external-unreachable=mermaid --assert-no-direct-import=src/web-ui/src/main.tsx:./tools --top=15
• Select-String -Path dist\index.html -Pattern 'mermaid|prism|react-syntax'
• git diff --check upstream/main..HEAD