Agent: resolve "today" in the browser's timezone, not hardcoded CR

The CR-hardcoded today broke two scenarios: the owner traveling, and
any future non-CR user. Now the provider sends the browser's IANA
timezone (X-Client-Timezone) with every CopilotKit request — the
runtime forwards x-* headers to the agent on its own — and the backend
binds it to a per-request ContextVar next to the DB session.
get_current_date and the future-cuota bounds use it; the tool also
reports the timezone so the model can echo it. Unknown or absent zones
(n8n posts, tests) fall back to Costa Rica; comma-joined duplicate
header values are tolerated.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/agent/agent.py

@@ -21,9 +21,10 @@ Context you can rely on:
   qualifiers, assume they mean the calendar month unless they mention
   "cycle", "corte", or their credit card.
 - You do NOT have a reliable clock, and the server runs in UTC while the
-  user lives in Costa Rica (UTC-6). Whenever the question involves a
-  relative date — "hoy", "ayer", "este mes", "este ciclo", "el año
-  pasado" — call get_current_date FIRST and derive ranges from its answer.
+  user may be anywhere (their browser reports their timezone per request).
+  Whenever the question involves a relative date — "hoy", "ayer", "este
+  mes", "este ciclo", "el año pasado" — call get_current_date FIRST and
+  derive ranges from its answer.
 - Amounts are stored as raw numbers in their native currency (see `currency`
   field on transactions/accounts). Tools that return `total_crc` are already
   converted; tools that return per-transaction amounts are NOT.