fix(room): display backend timestamps in the viewer's local zone

[91jaeminjo]

What

Backend timestamps arrive as UTC instants (parseTimestamp appends Z and calls .toUtc()). Two room display sites read calendar fields off those instants without .toLocal() first, so they rendered the wrong time-of-day (or a date off by one) for any viewer outside UTC:

• documents_card.dart _formatDateTime — full date and time, wrong hour on every render.
• thread_tile.dart _formatRelativeTime — the >7-day numeric-date fallback only.

Both bugs originated from copy-pasting a relative-time formatter that read raw fields, so the fix also removes that duplication.

Changes

• Fix: .toLocal() at both display sites.
• Consolidate: extract the duplicated relative-time label into a shared, already-localized formatRelativeTime in lib/src/shared/relative_time.dart, with .toLocal() normalized at the top of the function so the fallback can't regress. thread_tile, room_card, and room_grid_card all call it; the duplicate _formatRelativeTime and lobby's formatRelativeActivity are deleted.
• Kept in place: lobby's bucketFor / ActivityBucket (single consumer, lobby-specific section headers); documents_card's absolute formatter (different concept, single caller — not shared).

Design notes

shared/ (not core/) is the home: core/ is shell infrastructure, and a pure presentation helper shared by two sibling modules must live below both — room → lobby would be a wrong directional dependency. shared/ already holds the pure-function file_type_icons.dart.

Tests

• New test/shared/relative_time_test.dart — thresholds + a UTC-instant fallback regression case.
• New regression in documents_card_test.dart — the existing timestamp test used a local DateTime, so it never exercised the UTC→local path (confirmed RED before the fix, GREEN after).
• Caveat: both UTC regression tests can only fail on a non-UTC runner; the top-of-function normalization is the durable guard.

Full suite green, dart format clean, flutter analyze zero warnings.

Closes #338

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