[BUG] Cron expressions ignore schedule.TZ field, always evaluate in UTC

[AaronJny]

Description

CronSchedule has a TZ field (defined in the struct), but computeNextRun()
in pkg/cron/service.go ignores it entirely. All cron expressions are evaluated
in UTC, causing incorrect trigger times for non-UTC users.

Example: a cron expression 0 9 * * * (daily at 9:00) triggers at UTC 9:00 =
CST 17:00 for users in Asia/Shanghai.

Root Cause

computeNextRun() creates the reference time via time.UnixMilli(nowMS), which
is always UTC. The schedule.TZ field is never read or applied before calling
gronx.NextTickAfter().

Proposed Fix

1. Read schedule.TZ and convert the reference time to the target timezone
   before computing next tick.
2. Add a configurable default_timezone in tools.cron config for server-wide
   default.
3. Use 3-tier fallback: schedule.TZ > config default_timezone > "Asia/Shanghai".

Note: The final fallback to "Asia/Shanghai" could be time.Local instead.
Happy to adjust based on maintainer preference.

Environment

• PicoClaw: main branch (synced to PR fix: add HTTP status code check in BraveSearchProvider #1024)
• User timezone: Asia/Shanghai (UTC+8)
• LLMs tested: GPT-5.2, GPT-5.3-codex

[sipeed-bot]

@AaronJny Hi! This issue has had no activity for over 2 weeks, so I'm closing it for now to keep things tidy. If it's still relevant, feel free to reopen it anytime and we'll pick it back up.