Add uninstall option to CLI and GUI installers

[erikdarlingdata]

Problem

There is no way to fully uninstall the Performance Monitor Dashboard. The installer creates server-level objects that survive a database drop:

• 3 SQL Agent jobs (Collection, Data Retention, Hung Job Monitor)
• 2 Extended Events sessions (PerformanceMonitor_BlockedProcess, PerformanceMonitor_Deadlock)
• Server-side trace (LongQueries)
• The PerformanceMonitor database itself

Additionally, the existing clean install path only deletes 2 of 3 Agent jobs and doesn't clean up XE sessions.

Reported in #419.

Solution

1. Standalone SQL script (install/00_uninstall.sql) for SSMS users
2. CLI flag (--uninstall) on the existing installer
3. GUI button (Uninstall) in the installer GUI
4. Fix existing clean install to remove all 3 jobs + XE sessions

blocked process threshold (s) will NOT be reset (other tools may depend on it).