feat(gradebook): bounded grade entry, out-of-range flags, and save confirmation

[LWS49]

Summary

Hardens external grade entry and surfaces out-of-range data. Grade and max entry are capped at the decimal(5,2) ceiling at the input layer; negative manual entry is allowed (floor-at-zero presumes penalty columns) while non-numeric input is rejected at entry instead of being silently discarded. Over-max and below-zero grades show an inline indicator whose tooltip copy is weighted-view-aware. A persistent below-the-tabs banner aggregates how many grades across how many externals are outside their range. A successful inline edit now confirms with a persistent toast that echoes the student, assessment, and old-to-new grade.

Design decisions

• Inline max-marks editing was removed; max is editable only through the panel's Edit prompt - max is a column-definition value (the subheader), not per-student data, so it belongs in a deliberate dialog, and it is coupled to the "Cap at max" toggle which only the Edit prompt shows alongside it.
• Out-of-range is surfaced at distinct layers with distinct jobs: per-cell icons (locate) and the aggregate banner (review-before-export), each scoped to a bound flag that is on, since nothing is clamped when a flag is off. The import flow adds an entry-time Verify warning in a later PR.
• The save-confirmation toast is persistent (no auto-dismiss) and echoes the full mutation rather than a generic "Saved" - external grades flow to exported finals and the optimistic update has already discarded the old value from the cell, so the toast is the only in-session record of what changed and helps catch a row/column misclick. There is no undo button, which the toast wrapper does not support and the re-editable cell makes unnecessary.

Regression prevention

Covers: out-of-range computation, the OutOfRangeAlert component and the GradebookIndex summary, cap-at-two-decimals entry, below-zero and over-max icon presence/absence, weighted vs non-weighted tooltip copy, the read-only max cell, the active-cell accept/revert controls, and the save-confirmation toast (fires with student/assessment/old-to-new, does not fire on a no-op edit). No change to stored grades - all bound and warning behavior is read-time only. This PR is frontend-only.