[dashboard-components] Extract view-formatters helper

Summary

• extract shared custom-view aggregate formatting into src/view-formatters.js
• replace the inline metric aggregate branch in src/presenter.js with the shared helper
• add focused unit coverage for aggregate and numeric formatter edge cases and update the browser harness import rewrites

Duplication evidence

• pages/dashboard/src/presenter.js:527 previously inlined the full count / distinct-count / sum / mean / min / max aggregate formatting branch inside renderMetricView(...)
• pages/dashboard/src/presenter.js:636, pages/dashboard/src/presenter.js:683, and pages/dashboard/src/presenter.js:879 also reused the same numeric formatting helpers for chart point construction and text labels
• this run collapses that repeated helper logic behind one shared presentation-only module for future custom metric and chart slices

Call sites collapsed

• pages/dashboard/src/presenter.js:527 now calls formatAggregateValue(...) instead of rebuilding aggregate formatting inline
• pages/dashboard/src/presenter.js chart helpers now import the same shared formatNumber(...) and toNumber(...) functions from src/view-formatters.js
• pages/dashboard/test/e2e/smoke.spec.js browser harness now rewrites the new helper import so the same presenter behavior is exercised in browser tests

Tests added

• pages/dashboard/test/unit/view-formatters.test.js
  • aggregate formatting for count, distinct-count, sum, mean, min, max, and default field access
  • empty, missing-field, and non-numeric inputs
  • deterministic shared numeric helper output

Behavior preservation proof

• preserved existing DOM text, class names, and custom-view metric/chart behavior by extracting only formatting logic
• compared the committed change scope with git diff --stat HEAD~1 HEAD and confirmed the affected files are limited to the presenter helper extraction, its tests, the Playwright module-rewrite harness, and PLAN.md
• reran the existing presenter unit suite and browser suite after wiring the new module into the Playwright data-URL harness

Quality gates

Ran from pages/dashboard/:

• npm install ✅
• npm run build ✅
• npm run typecheck ✅
• npm run lint ✅
• npm test ✅
• npx playwright test --config=playwright.config.mjs ✅

Blockers

• None

Next candidates

• extract the repeated custom-view state-message plus affected-source/context composition in src/presenter.js
• extract shared link-cell composition across custom metric and custom table views
• extract shared chart-series grouping and textual legend helpers if upcoming feature slices add more chart variants

network:
  allowed:
    - defaults
    - "mtalk.google.com"

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