feat(race-hub): trustworthy defaults and fan-facing analysis hierarchy (#75)

Make bare /race-hub resolve to a completed session and never open empty
post-session analysis for a future race.

- Backend: add `default_analysis_session` to the Weekend context. It never
  resolves to a future session (picks the richest completed session, ties
  toward the later one; 0 when everything is upcoming). Existing
  `default_session_key` and deep links are unchanged.
- Frontend default resolution prefers the most recently completed weekend
  (`pickAnalysisFocusMeeting`) and consumes `default_analysis_session`, falling
  back to the switcher when only upcoming sessions exist.
- New `sessionState` lib maps timing + coverage to user language
  (upcoming/live/preparing/partial/ready/cancelled); the session rail, active
  sub-bar, and WeekendSwitcher now label states instead of raw x/11 counts.
- Future sessions render a purpose-built PreSessionView (expected availability +
  countdown) instead of empty Winner/Podium/Pole/Strategy/Compare cards.
- Analysis navigation regrouped into Story / Analysis / Data & Context; every
  existing tab is preserved. Diagnostics (renamed from Data Status) is now a
  secondary action and the raw dataset strip is hidden behind an explicit
  toggle, so operational coverage no longer precedes fan content.
- Loading/error states offer Retry and a path back to Weekend.

Tests: Go query tests for future-exclusion; Vitest for default selection,
future pre-session, partial state, error/retry, grouped nav, and sessionState;
hermetic Playwright for bare/completed/future/return-to-Weekend; new
race-hub-future visual snapshots. Seed adds a far-future session inside the
Monaco meeting (kept in-meeting so Command Center focus is unaffected).

Note: `default_analysis_session` is an additive field on the existing
`/weekend` contract (no new endpoint), per the spec's "context contract" scope.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/styles/app.css

@@ -1820,6 +1820,141 @@ a { color: inherit; text-decoration: none; }
 .tab-btn:hover { color: var(--text-2); }
 .tab-btn.active { color: var(--text); border-bottom-color: var(--red); }
 
+/* ── Grouped analysis navigation ── */
+.tab-bar-grouped {
+  gap: var(--s5);
+  align-items: flex-end;
+}
+.tab-group {
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+  flex-shrink: 0;
+}
+.tab-group + .tab-group {
+  border-left: 1px solid var(--border);
+  padding-left: var(--s5);
+}
+.tab-group-label {
+  font-size: 9px;
+  letter-spacing: 0.12em;
+  text-transform: uppercase;
+  color: var(--text-3);
+  opacity: 0.7;
+  padding: 0 14px;
+}
+.tab-group-btns {
+  display: flex;
+}
+
+/* ── Session lifecycle state dots ── */
+.rh-state-ready { background: var(--green); }
+.rh-state-partial { background: var(--yellow); }
+.rh-state-live { background: var(--red); }
+.rh-state-upcoming { background: var(--text-3); }
+.rh-state-preparing { background: var(--text-3); opacity: 0.5; }
+.rh-state-cancelled { background: var(--text-3); opacity: 0.35; }
+
+/* ── Pre-session (expected availability) view ── */
+.rh-presession {
+  display: flex;
+  flex-direction: column;
+  gap: var(--s5);
+}
+.rh-presession-band {
+  padding: var(--s6) var(--s5);
+  background: var(--surface);
+  border: 1px solid var(--border);
+  border-left: 3px solid var(--gp-accent);
+  border-radius: 4px;
+  display: flex;
+  flex-direction: column;
+  gap: var(--s3);
+}
+.rh-presession-eyebrow {
+  font-size: 10px;
+  letter-spacing: 0.14em;
+  text-transform: uppercase;
+  color: var(--text-3);
+}
+.rh-presession-title {
+  font-size: 22px;
+  font-weight: 700;
+  margin: 0;
+}
+.rh-presession-sub { color: var(--text-2); max-width: 60ch; }
+.rh-presession-countdown {
+  font-size: 15px;
+  color: var(--text);
+  margin-top: var(--s3);
+}
+.rh-expected-grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fill, minmax(160px, 1fr));
+  gap: var(--s4);
+}
+.rh-expected-card {
+  padding: var(--s4);
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  background: var(--surface);
+  display: flex;
+  align-items: center;
+  gap: var(--s3);
+  color: var(--text-2);
+  font-size: 13px;
+}
+.rh-expected-card .rh-expected-dot {
+  width: 6px;
+  height: 6px;
+  border-radius: 50%;
+  background: var(--text-3);
+  opacity: 0.6;
+}
+
+/* ── Loading / error recovery ── */
+.rh-recover {
+  display: flex;
+  flex-direction: column;
+  align-items: flex-start;
+  gap: var(--s4);
+  padding: var(--s6) var(--s5);
+}
+.rh-recover-actions {
+  display: flex;
+  gap: var(--s3);
+  flex-wrap: wrap;
+}
+.rh-recover-btn {
+  padding: 8px 14px;
+  font-size: 12px;
+  font-weight: 600;
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  background: var(--surface);
+  color: var(--text);
+  cursor: pointer;
+  text-decoration: none;
+}
+.rh-recover-btn:hover { border-color: var(--gp-accent); }
+.rh-recover-btn.primary { border-color: var(--red); color: var(--text); }
+
+/* ── Diagnostics (secondary) action ── */
+.rh-diagnostics-toggle {
+  align-self: flex-start;
+  margin-left: auto;
+  padding: 6px 12px;
+  font-size: 11px;
+  font-weight: 600;
+  color: var(--text-3);
+  background: none;
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  cursor: pointer;
+}
+.rh-diagnostics-toggle:hover { color: var(--text-2); border-color: var(--gp-accent); }
+.rh-diagnostics-toggle.active { color: var(--text); }
+
 /* ── Dataset status view ── */
 .ds-legend {
   display: flex;