fix: invalid frame index calculation can seek to -1 or out-of-range frames

[Nam0101]

Description

get_video_frame() computes the seek position as min(frame_total, frame_number - 1). With the default frame_number=0, this becomes -1 (invalid frame index). For large requested frame numbers, it can also become frame_total (also invalid, since last valid frame is frame_total - 1). This causes intermittent failed reads (has_frame=False) and breaks callers that expect a valid first frame by default.

Severity: high
File: modules/capturer.py

Expected Behavior

The code should handle this case properly to avoid unexpected errors or degraded quality.