feat(gradebook): By-weight view with Configure Weights dialog

[LWS49]

Summary

• Adds "By weight" gradebook view: per-tab subtotal columns + weighted Total column, behind a [All assessments | By weight] toolbar toggle (rendered only when the course-level setting from PR 1 is enabled)
• 3-row sticky header: categories → tab titles → "X% of grade" weight subheaders
• Configure Weights dialog for managers: per-tab integer input, live sum, warning when sum ≠ 100 (save allowed)
• "Treat Ungraded as 0" toolbar toggle — client-side, recalculates subtotals and total live
• Total subheader shows warning icon when tab weights don't sum to 100
• Empty-state banner when all tab weights are 0

Architecture

• Pure compute helpers (computeWeighted.ts) — computeTabSubtotal, computeStudentTotal, sumWeights
• Redux store extended: weightedViewEnabled, canManageWeights, gradebookWeight per tab; updateTabWeights action
• GradebookWeightedTable — self-contained table with custom 3-row thead; no column picker (all subtotals always visible)
• ConfigureWeightsDialog — dispatches updateGradebookWeights thunk → PATCH /gradebook/weights
• GradebookIndex — reads weightedViewEnabled selector; conditionally renders ToggleButtonGroup + swaps table component