Add API config persistence, thumbnails, and task logging

[shensz2017]

Motivation

• Persist API credentials locally so users don't need to re-enter API Key and Base URL every launch.
• Improve UX by showing uploaded image thumbnails in the task list for easier identification.
• Surface request and download outcomes in the UI via a task log instead of only printing to stdout.

Description

• Added a config.json file path (self.config_path) and implemented load_api_config and save_api_config with a 💾 保存 API Key button to persist base_url and api_key to disk and load them at startup.
• Introduced a read-only QTextEdit task log (self.log_output) and an append_log helper to write timestamped messages for load/save, submit, failures, and download completion.
• Added thumbnail support by storing image_path on task create and rendering scaled QPixmap thumbnails in a new Image column in the tasks table, and adjusted the table from 6 to 7 columns and moved the action widgets accordingly.
• Enhanced submit and polling flows to append relevant messages to the log (submit failures, request failures, task failures, and download successes) and to record task url and local_path as before.
• Imported QPixmap and minor UI tweaks for row height and button wiring to support the new features.

Testing

• No automated tests were run for this change (not run).
• Manual code inspection and local commits were used to verify the UI changes compiled into the single modified file sora2.py.

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Codex Task