i18n: make chart tooltip dates use the app locale (formatShortDate)

[NesiciCoding]

Deferred from PR #363 (CodeRabbit review: #363 (comment)).

Problem

formatShortDate(iso) (src/utils/dateInput.ts) formats via the browser locale (toLocaleDateString() with no locale argument), so tooltip dates can differ from the user's selected app language. This surfaced in the Statistics trend-chart tooltips:

• src/components/Statistics/ClassTrendChart.tsx (tooltip label formatter)
• src/components/Statistics/MultiClassTrendChart.tsx (tooltip label formatter)
• other formatShortDate callers repo-wide

Why deferred

formatShortDate is a shared utility used across many charts/exports. Making it locale-aware is a cross-cutting change best done as one consistent pass (util signature + all callers) rather than a one-off in the two trend tooltips.

Suggested work

• Add an optional locale parameter to formatShortDate (e.g. formatShortDate(iso, locale?)), passing it through to toLocaleDateString(locale, ...).
• Update callers that render user-visible dates to pass the active i18n.language.
• Confirm no regression in export utilities that call it.

Scope: small–medium. Non-blocking; no functional bug, just locale consistency.

[NesiciCoding]

Partially addressed in #383: formatShortDate(iso, locale?) now takes an optional locale, and 7 user-visible callers pass i18n.language (Dashboard, LearningGoalChart, StudentPortalPage, StudentProfilePage ×2, StudentCefrOverviewPage).

Remaining — the two charts this issue names cannot be fixed on main yet. ClassTrendChart / MultiClassTrendChart gain their tooltip formatShortDate(d) calls only in the unmerged phase-37.2 stack (#361), where they still pass no locale:

labelFormatter={(label, payload) => `${label} · ${formatShortDate(d)}`}  // no i18n.language

Those call sites don't exist on main, so #383 (branched off main) can't touch them. Follow-up once #361 lands: add , i18n.language to both labelFormatter calls. Keep this issue open until then.

[NesiciCoding]

Now fully addressed in #383. The phase-37.2 stack (#361) landed on main, so this branch was rebased onto it and the ClassTrendChart / MultiClassTrendChart tooltip formatShortDate calls (plus the new Students "last active" column) now pass i18n.language. Every user-visible formatShortDate call is locale-aware; closing via #383.