fix(widgets): stop rendering missing data as if it were real (#568) (#575)

* fix(widgets): stop rendering missing data as if it were real (#568)

Three widget states presented absent data as a value a teacher could act
on, plus three adjacent defects found while fixing them.

A student with no `profiles.full_name` rendered as a slice of their user
id — `u-008` in the name column, `U-` in the avatar. That is not a name,
identifies nobody, and reads as corrupted data. `student_name: null` is a
normal value (fetchProfileNames drops blanks), so the fallback now goes
through a shared `resources/shared/student-display.ts`: "Unnamed student"
in a muted italic that reads as a placeholder, and a neutral avatar glyph
instead of letters derived from an id. `submission-grader` had the same
`student_id.slice(0, 8)` fallback; `exam-submissions` declared
`student_name` non-nullable while its tool sends null, so a nameless
student rendered as an empty cell.

The roster showed "1 exam" beside an em dash — the same glyph it uses for
"no data" elsewhere — so an ungraded submission read as an exam scored
nothing. It now reads "Ungraded". That state was also unreachable from
real data: `exam_count` was incremented only `if (score != null)`, which
made an ungraded submission vanish entirely and the student read as "0
exams". The aggregation is extracted as `aggregateExamSubmissions()` and
now counts every submission while averaging only the graded ones.

The course dashboard blamed a filter for every empty result, including
the first-run case where no filter exists, the chips have collapsed to a
lone "All", and the teacher is offered no way forward. It now branches:
no courses at all (first-run copy plus a create-course action, chips
hidden), no courses of a server-filtered status, a chip that matched
nothing, and a page past the end of the list. That last case is newly
reachable because `lms_list_courses` was reporting a hardcoded
`total: 0` on an empty page, discarding the real count — which would have
shown a teacher with courses the "create your first course" screen.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VSGmz5frZqzbpQXJxxuqTs

* docs(qa): before/after widget evidence for #568

Screenshots and A/B GIFs backing PR #575. Kept on the branch rather than
master so the PR carries its own evidence; safe to drop before merge.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VSGmz5frZqzbpQXJxxuqTs

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: guillermoscript