Fix browser-timezone session date filters

[gtkacz]

Session list and sidebar calendar-date filters now accept the browser IANA timezone, default omitted values to UTC, and reject invalid zones.

Local calendar dates are translated into DST-aware UTC boundaries while preserving latest-message activity fallback and equivalent SQLite, PostgreSQL, and DuckDB query behavior. The sidebar sends the browser timezone on initial and paginated requests, and the generated API client reflects the new parameter.

Fixes #1232.

[roborev-ci]

roborev: Combined Review (4ef2093)

Verdict: One medium-severity cursor consistency issue must be fixed.

Medium

• internal/db/session_export.go:125 — Timezone is omitted from sessionExportCursorFilters and its conversion functions. Resuming with UseCursorFilter silently falls back to UTC, and timezone changes between pages are not detected as filter conflicts, potentially causing missing or duplicate export rows.

  Fix: Add Timezone to the cursor payload, canonicalization, and both filter conversion paths. Add tests covering cursor resume and timezone changes.

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Reviewers: 2 done | Synthesis: codex, 10s | Total: 7m13s

[roborev-ci]

roborev: Combined Review (9032e2b)

No issues found.

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Reviewers: 2 done | Synthesis: codex | Total: 7m14s