feat(gradebook): add weighted view with per-tab grade weights (pr3 of 3)

[LWS49]

Summary

Adds a "By weight" view to the staff gradebook so course managers can assign per-tab grade weights and see weighted subtotals and a Total column per student. The view is gated behind a course-level setting toggled from Course Admin → Gradebook. When enabled, a toggle group appears in the gradebook toolbar; the default view (All assessments) is unchanged.

Managers can open a Configure Weights dialog to assign integer weights (0–100) per tab. Weights do not need to sum to 100 — the Total is normalised to whatever the weights sum to, and a warning is shown when the sum is off. A "Treat Ungraded as 0" toggle adjusts how missing submissions are counted, matching the Canvas-style convention. TAs see the weighted view and subtotals but not the Configure Weights button.

This is PR 3 of 3. PR 1 added the gradebook_weight column, weighted_view_enabled course setting, and abilities. PR 2 exposed those via the gradebook JSON and added the PATCH /gradebook/weights endpoint.

Design decisions

• Weights normalise, not clamp to 100 - saving weights that do not sum to 100 is allowed; the Total divides by actual weight sum rather than 100. This avoids blocking saves mid-edit and matches how most LMSes handle partial configurations.
• "Treat Ungraded as 0" is view-only, not persisted - the toggle only affects the current browser session. Persisting it would require a user preference store; the spec explicitly deferred that.
• Compute helpers are pure functions in a separate module - computeTabSubtotal and computeStudentTotal live in computeWeighted.ts with no Redux dependencies, making them independently testable and reusable.
• ConfigureWeightsDialog calls dispatch(updateGradebookWeights(...)) not the thunk factory directly - the thunk pattern requires dispatch; calling the factory without it silently discards the API call (caught and fixed in review).

Regression prevention

Tests added:

• computeWeighted.test.ts - covers null propagation (empty tab, no grades, toggle off), ratio calculation, ungraded-as-zero, weighted average, weight-0 exclusion, normalisation when weights ≠ 100
• ConfigureWeightsDialog.test.tsx - covers rendering, live sum, sum ≠ 100 warning, inline errors for out-of-range and non-integer values, Save dispatch, Cancel no-op
• GradebookWeightedTable.test.tsx - covers 3-row header structure, Total column warning, per-student subtotals, tab with no assessments (dash), Treat Ungraded as 0 recompute, all-weights-zero empty state, canManageWeights gate on Configure Weights button
• GradebookIndex.test.tsx (+3) - toggle hidden when setting off, toggle shown when on, By weight view mounts on click
• All 77 gradebook tests pass; no regressions in existing table, column-picker, or CSV tests

Backward compatibility - the "All assessments" view and all existing gradebook behaviour are unchanged. The weighted view is only reachable when the course-level setting is explicitly enabled.

Manual testing checklist (to be run before merge):

• Enable "Calculate total by tab weights" in course admin settings as manager; reload and confirm it persists; TA/student hitting /admin/gradebook is blocked
• Navigate to gradebook - "All assessments | By weight" toggle appears; default is All assessments (existing view unchanged)
• Switch to By weight → click Configure Weights → enter weights (e.g. 60 / 40) → Save → subtotal columns and Total column populate with correct percentages
• Set weights to 60 / 30 → warning in dialog and "90% total ⚠" in Total column subheader
• Set all weights to 0 → empty-state banner appears; Total shows "—"
• Flip "Treat Ungraded as 0" → ungraded submissions count as 0 toward denominator; numbers change; flip back and numbers restore
• Sign in as TA → weighted view and subtotals visible; Configure Weights button absent
• Disable setting in admin → gradebook shows All assessments only with no toggle; re-enable → weights still intact