fix(detail): gap-honest year axis on the detail-page trend charts (Batch D)

Applies the compare screen's chart-truthfulness rules (spec §8.1) to the
school detail page's time-series charts.

PerformanceChart (Results Over Time, both phases):
- Fill every academic year between the first and last data point via the
  shared fillAcademicYears helper, so cancelled/unpublished years (2019/20,
  2020/21, and — for KS2 — 2021/22) render as real gaps instead of
  compressed time. Each series and the England overlay map onto this filled
  axis with null for missing years; spanGaps:false so school lines never
  bridge a gap.
- Replace the primary-only COVID note with a distinct, honest gap caption:
  KS2 names the cancelled tests plus the unpublished 2021/22 school-level
  year; KS4 names the unpublished 2019/20–2020/21 GCSE grading years.
- Mobile x-axis switches to autoSkip so the longer (gap-honest) axis stays
  readable; the broken line still marks a missing year even when its tick
  label is skipped.

AdmissionsTrendChart:
- Same gap-honest axis + spanGaps:false so a missing admissions year is a
  real gap, not compressed time.

Point 12 (definite canvas heights): desktop is already a definite 280px;
fix the secondary detail's mobile .chartContainer, which fixed the outer
box at 220px and double-constrained PerformanceChart's own 220px canvas +
chip strip (clipping the chips onto the plot) — now height:auto to match
the primary view.

SatsChart is out of scope (single-year per-subject CSS bars — no year axis,
no canvas height to constrain).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

nextjs-app/components/SecondarySchoolDetailView.module.css

@@ -1049,8 +1049,12 @@
         font-size: 1.85rem;
     }
 
+    /* On mobile let the chart container flow naturally — PerformanceChart's
+       own .chartWrapper carries the definite canvas height (220px) plus the
+       chip strip above it. A fixed 220px here double-constrained the two and
+       clipped the chips onto the plot area. */
     .chartContainer {
-        height: 220px;
+        height: auto;
     }
 
     .dataTable {