feat(gradebook): import did-you-mean, conflict change-matrix, loading cues, and copy polish

[LWS49]

Summary

Wizard UX for the external grade importer. The Verify step shows header mismatches with did-you-mean suggestions and a malformed-cell list, and identifies rows by their External ID with a resolved-name column for match confirmation. The conflict prompt is redesigned as a per-student matrix showing each changed cell old → new, dropping unchanged rows. Adds loading affordances, scroll handling for wide previews, typography fixes, a reassigned-identifier advisory, and copy polish throughout.

Design decisions

• Conflicts are shown as a per-student change matrix with unchanged cells dropped - the previous flat list re-counted cells whose value was identical at two decimals; the matrix shows only real changes (old → new) grouped by the student they affect.
• Identifier-first display (External ID with a resolved-name column) replaces name-only rows - under an identifier-mode header, showing only the resolved name read as a mismatch; pairing the CSV value with the resolved name confirms the match.

Regression prevention

Covers: the import wizard (template/roster/from-existing, out-of-range verify, did-you-mean, loading), and the conflict change-matrix table (struck-old/arrow/bold-new, em-dash for null sides and missing cells, new-fill vs existing fallback, header/row identity) (frontend suites green). No backend logic change (that is PR4a). Import-flow specs run on push/CI.

Screenshots worth adding: the redesigned conflict change-matrix, and the Verify step with a did-you-mean suggestion.

[LWS49]

Superseded by #8465 (import correctness + UX consolidated into a single import PR).