fix(blog): pin every date formatter to UTC

[schmug]

Closes #222.

Every Intl.DateTimeFormat in src/ now carries timeZone: 'UTC', so a post's rendered date is a property of its content rather than of the machine that built the page.

The bug

Blog frontmatter carries a bare pubDate: YYYY-MM-DD, which z.coerce.date() parses as UTC midnight — the single value guaranteed to land on the previous day in every western zone. The formatters had no timeZone, so every blog post rendered a day early on a negative-offset builder, with the visible text contradicting the <time datetime> attribute in the same element. Production is correct today only because Cloudflare Pages happens to build in UTC.

BlogApp.tsx and Timeline.tsx render client-side, where the unpinned zone is the viewer's — a reader in Los Angeles saw a different day than one in Berlin for the same post.

Scope note

The issue lists seven sites; this changes eight. src/components/mythos/Timeline.tsx:44 formats bare YYYY-MM-DD history rows in the viewer's zone — same defect class, and acceptance criterion 4 ("no Intl.DateTimeFormat without an explicit timeZone remains in src/") covers it.

Display-only. Stored pubDate values, datetime attributes, sort order (on timestamps, already zone-independent) and the RSS RFC-822 output are untouched; each formatter keeps its existing short/long month style. No new dependencies.

Evidence

Podcast and frontier-commits render an empty state locally (EPISODES_MANIFEST_URL unset), so those paths were exercised with synthetic manifests fed through EPISODES_MANIFEST_URL / FRONTIER_MANIFEST_URL carrying boundary-hugging stamps — 01:00Z (previous day in EDT) and 23:30Z (next day in Tokyo). 16 <time> elements across dist/blog, dist/podcast, dist/frontier-commits.

Criterion 2 — visible text vs. the calendar date of its own datetime attribute:

build	parsed	mismatches
pre-fix, TZ=America/New_York	16	12
post-fix, TZ=America/New_York	16	0

Pre-fix sample:

MISMATCH: 2026-04-16T00:00:00.000Z -> "Apr 15, 2026"   | UTC calendar date is Apr 16, 2026
MISMATCH: 2026-04-18T00:00:00.000Z -> "April 17, 2026" | UTC calendar date is Apr 18, 2026
MISMATCH: 2026-05-23T01:00:00.000Z -> "May 22, 2026"   | UTC calendar date is May 23, 2026

Criterion 1 — three full builds, rendered date strings extracted and sorted:

America/New_York:  16 <time> elements
UTC:               16 <time> elements
Asia/Tokyo:        16 <time> elements

diff EDT vs UTC    -> IDENTICAL
diff UTC vs Tokyo  -> IDENTICAL

Criterion 3 — BlogApp.tsx is client-side, so no build check can prove it. New BlogApp.test.tsx renders the app under America/Los_Angeles, UTC and Asia/Tokyo (the formatter is module-level, so vi.resetModules() rebuilds it per zone) and asserts Apr 16, 2026 in all three. Confirmed failing for the right reason with the pin removed:

AssertionError: expected 'Apr 15, 2026' to be 'Apr 16, 2026'
Tests  1 failed | 2 passed (3)

Criterion 4 — grep -rn "Intl.DateTimeFormat" src returns 11 matches; all 11 carry timeZone: 'UTC'.

Criterion 5 — full gate suite on the rebased branch:

format:check  All matched files use Prettier code style!
lint          clean
typecheck     131 files: 0 errors, 0 warnings, 1 hint (pre-existing)
test          34 files, 297 tests passing
build         26 pages built

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