[Core] Auto-update check on startup

[HuckleR2003]

Milestone: v1.7.0
Goal: Check GitHub releases API on startup, show banner if newer version available.

Tech:

• GET https://api.github.com/repos/HuckleR2003/PC_Workman_HCK/releases/latest
• Compare installed version (from version.py) vs latest tag
• Show dismissible banner in dashboard if newer found
• Click -> open release page in browser
• Cache check (max 1x per day to avoid API spam)

Implementation:

• New module: core/update_checker.py
• Non-blocking (thread/async, don't delay startup)
• Graceful fail if no internet (silent, no error popup)
• User setting: "Check for updates" toggle (default: ON)

Success criteria:

• Check completes in <500ms
• Banner shows for new releases
• No crashes if GitHub API down
• Users find updates without Twitter/LinkedIn

NOT doing:

• Auto-download (security risk)
• Forced updates (annoying)
• Telemetry (privacy)