GPT : Timezone and reporting improvements

Add timezone-aware timestamp support and reporting improvements
Store timestamps in ISO-8601 format with timezone offsets
Accept client-local timestamps from browser and API
Normalize legacy naive timestamps during parsing
Add migration script for converting old SQLite timestamps
Fix mixed naive/aware datetime comparison errors
Render timestamps in browser-local timezone
Auto-generate default 7-day reports on page load
Expand default dashboard report to include all users
Preserve manual single-user report generation
Add curl-based validation/test script for API and report flows

migrate_db.sh

@@ -0,0 +1,99 @@
+#!/usr/bin/env bash
+
+#
+# SQLite Timestamp Migration
+#
+# Converts legacy naive timestamps:
+#
+#   2026-05-29 08:15:00
+#
+# into timezone-aware ISO-8601 timestamps:
+#
+#   2026-05-29T08:15:00-04:00
+#
+# Existing timezone-aware timestamps are preserved.
+#
+
+set -euo pipefail
+
+DATABASE="${1:-database/timeclock.db}"
+
+#
+# Timezone offset to apply to old naive timestamps.
+#
+# Examples:
+#   -04:00   Eastern Daylight Time
+#   -05:00   Eastern Standard Time
+#   +00:00   UTC
+#
+DEFAULT_OFFSET="-04:00"
+
+echo "========================================="
+echo "Timeclock Timestamp Migration"
+echo "========================================="
+echo
+echo "Database: ${DATABASE}"
+echo "Assumed legacy offset: ${DEFAULT_OFFSET}"
+echo
+
+if [ ! -f "${DATABASE}" ]; then
+    echo "ERROR: Database not found"
+    exit 1
+fi
+
+BACKUP="${DATABASE}.backup.$(date +%Y%m%d%H%M%S)"
+
+echo "Creating backup:"
+echo "  ${BACKUP}"
+
+cp "${DATABASE}" "${BACKUP}"
+
+echo
+echo "Beginning migration..."
+echo
+
+sqlite3 "${DATABASE}" <<SQL
+BEGIN TRANSACTION;
+
+--
+-- Convert naive timestamps to ISO8601 with timezone
+--
+-- Only modify timestamps that:
+--   1. Do NOT already contain timezone offsets
+--   2. Do NOT already contain a trailing Z
+--
+
+UPDATE entries
+SET ts =
+    REPLACE(ts, ' ', 'T') || '${DEFAULT_OFFSET}'
+WHERE
+    ts NOT LIKE '%+__:__'
+    AND ts NOT LIKE '%-__:__'
+    AND ts NOT LIKE '%Z';
+
+COMMIT;
+SQL
+
+echo
+echo "Migration complete."
+echo
+
+echo "Verifying results..."
+echo
+
+sqlite3 "${DATABASE}" <<SQL
+.headers on
+.mode column
+
+SELECT
+    id,
+    user_id,
+    entrytype,
+    ts
+FROM entries
+ORDER BY id DESC
+LIMIT 10;
+SQL
+
+echo
+echo "Done."