feat: times in the reader's own zone; the audit log shows exact moments (milestone 490)

- users.time_zone (0034). An account without one takes the browser's zone
  on its first page (POST /account/time-zone/adopt, never over a choice made
  by hand); the session follows the account on every request.
- /account: who you are and the zone every page writes its times in,
  reached from your name in the sidebar.
- formatting.when: 'Today 06:58', 'Yesterday 23:41', 'Tue 6 Oct 14:12',
  '6 Oct 2025 14:12', 24-hour, in the reader's zone; |ago's hover is the
  exact local moment instead of UTC; tz_label() for column headers.
- Audit log: When (EDT) with exact times, one detail fact a line, on classes.
- tzdata in the image, and the airgapped smoke checks a zone resolves.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/app.py

@@ -138,7 +138,15 @@ def create_app(
     app.jinja_env.filters["bytes"] = _fmt.human_bytes
     app.jinja_env.filters["rate"] = _fmt.human_rate
     app.jinja_env.filters["duration"] = _fmt.human_duration
-    app.jinja_env.filters["ago"] = _fmt.ago
+    # A moment is written in the reader's zone, saved on their account and held
+    # in the session (milestone 490); outside a request (an email, a report)
+    # it is UTC.
+    def _viewer_zone():
+        from quart import has_request_context, session
+        return _fmt.zone(session.get("tz")) if has_request_context() else _fmt.zone(None)
+    app.jinja_env.filters["ago"] = lambda moment, now=None: _fmt.ago(moment, now, tz=_viewer_zone())
+    app.jinja_env.filters["when"] = lambda moment: _fmt.when(moment, _viewer_zone())
+    app.jinja_env.globals["tz_label"] = lambda: _fmt.tz_label(_viewer_zone())
     app.jinja_env.globals["status_class"] = (
         lambda value, kind: _fmt.status_class(value, kind, app.config.get("THRESHOLDS", {}))
     )
@@ -211,6 +219,7 @@ def create_app(
     from .settings.routes import settings_bp
     from .audit.routes import audit_bp
     from .ai_access.routes import ai_access_bp
+    from .account.routes import account_bp
 
     app.register_blueprint(auth_bp)
     app.register_blueprint(dashboard_bp)
@@ -223,6 +232,7 @@ def create_app(
     app.register_blueprint(settings_bp)
     app.register_blueprint(audit_bp)
     app.register_blueprint(ai_access_bp)
+    app.register_blueprint(account_bp)
 
     # Register the unified Monitor status source for the Status page. Plugins
     # may register additional sources from setup(). Idempotent.