feat(smartem): /models heatmap matrix on mock data (PR 67a)

[vredchenko]

Summary

PR 67a from the plan in #67. Adds the model × metric weights heatmap to apps/smartem at the existing top-level /models nav route. Renders on a hand-rolled mock fixture; no chart library introduced. apps/legacy is untouched.

• New route apps/smartem/src/routes/models.tsx — page header + matrix.
• New component apps/smartem/src/components/weights/WeightsMatrix.tsx — CSS-grid layout, MUI Tooltip per cell, gradient legend below.
• New colour helper apps/smartem/src/components/weights/cellColor.ts — single-hue blue ramp, dark-text-on-light / light-text-on-dark above 0.55.
• New mock fixture apps/smartem/src/data/mock-model-weights.ts — 4 models × 5 metrics = 19 populated cells + 1 empty for the "no data" demo. Shaped to match orval's QualityPredictionModelWeight type so PR 67b2 can swap in useGetModelWeightsForGridGridGridUuidModelWeightsGet without restructuring the data flow.
• Auto-regenerated apps/smartem/src/routeTree.gen.ts for the new /models route.

What's deferred

• Chart library / sparklines / time series: blocked on the time-series UI library ADR tracked at smartem-devtools#196. PR 67b2 lands after that ADR.
• Time slider: PR 67c.
• Grid / acquisition selector: MVP hard-codes a mock grid id. Page header surfaces the temporary scope ("weights for grid-A1 (mock)").

Test plan

• npm run typecheck — clean across all workspaces
• npm run check (Biome lint + format) — clean
• Pre-push hooks green (typecheck, lint, format-check, gitleaks)
• Smoke check via npm run dev:smartem:mock (Playwright):
  • Page header "Models" + subheader "weights for grid-A1 (mock)"
  • 4 row headers (resnet-atlas, vit-gridsquare, efficientnet-ice, dae-atlas), 5 column headers (astigmatism, ctf_resolution, defocus, ice_thickness, motion)
  • 19 weight cells with values (e.g. 0.82, 0.30) colour-coded; 1 empty — cell
  • Hover tooltip on 0.82 cell: resnet-atlas · defocus | Weight: 0.820 | Apr 29, 2026, 07:00 PM
  • Legend gradient with stops 0.00 / 0.25 / 0.50 / 0.75 / 1.00
  • Zero console errors

Closes none — progresses #67.