perf: lazy-load posthog-js (#406) and date-fns locales (#408) - #46

date-locale.ts statically imported all 12 date-fns locales from the barrel
(~97 KB raw / ~19 KB brotli), so every visitor downloaded every locale though
only one is ever used. Now en-US is the static default/fallback and the other 11
are lazy chunks (one per locale, literal import() specifiers) loaded on demand.

Consumers need a Locale synchronously in render (incl. non-hook utils time.ts and
format-label-date.ts), so the locale is preloaded BEFORE the language becomes
active: preloadDateFnsLocale() is awaited immediately before i18n.changeLanguage()
at the two (and only two) app-language set points — user-provider.tsx and
account-language.tsx. getDateFnsLocale() therefore stays a pure synchronous getter
that is always correct, with zero flash and no subscription store / event re-emit.
Loaders are best-effort (a failed chunk keeps the en-US fallback and never
rejects). Public API (getDateFnsLocale/useDateFnsLocale/formatLocalized) and en-US
output are unchanged.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

…al-JS budget

Add "posthog-js" to mustStayLazy (matches the core and its /react subpath under
node_modules/) so a future eager re-import reds the strict build. With posthog-js
(docmost#406) and 11 date-fns locales (docmost#408) off the eager graph, the initial JS set
dropped 789,788 -> 722,673 B brotli; tighten the budget ceiling 860,000 -> 790,000
(~9% headroom) to lock in the win. CSS unchanged (48,435 B).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

…t the locale-switch invariant

Extract changeAppLanguage() in date-locale.ts (preload the date-fns locale, then
i18n.changeLanguage) and route both app-language switch sites (user-provider,
account-language) through it. This turns the previously-unenforced "every
changeLanguage must be preceded by preloadDateFnsLocale" invariant into an
enforceable, testable one (Architecture + Testing L3):

- date-locale.test.ts: source-scan fitness function — no client file calls
  i18n.changeLanguage() outside date-locale.ts, and both switch sites use the
  helper (mirrors lazy-pages.test.ts); an ordering test proving the locale is
  cached BEFORE the switch fires (a change-then-load reorder fails it); a
  formatLocalized non-English test (the localized-pattern branch was unguarded);
  order-independent fallback assertions (reserved ko-KR/it-IT locales).

Also: reword the overstated posthog-init.ts / posthog-analytics.test.tsx comments
(posthog-js has two static importers, both lazy-reached; the off-eager-graph
guarantee is the budget lane's, not the effect test's); reconcile the 2026-09-18
baseline to 789,788 B across vite.config.ts.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

…osthog-datefns