Add click circle effects

[AaravKhanduja]

Adds animated click circle effects at mouse click positions during playback and export.

Changes

• Click circle rendering in both canvas preview and Sharp-based export
• Properties Panel UI: toggle, color picker, size slider
• Shared DEFAULT_EFFECTS constant for consistent defaults
• Defensive deep-merge for missing/partial effects metadata
• Keyframe interval fix for reliable MKV seeking

Test plan

• Toggle click circles on/off in Properties Panel
• Circles render at correct click positions in preview
• Circles appear in exported video with cursor still visible
• Correct positions with zoom enabled
• Old recordings without effects metadata load without crash