fix(demo): align frame naming between capture and encoder

[gregpriday]

Summary

• The capture pipeline wrote frames as frame-000001.png (hyphen, 6-digit) while the encoder regex and ffmpeg pattern both expected frame_0001.png (underscore, 4-digit), so encode() always threw immediately after a real capture run
• Updated the encoder regex, ffmpeg input pattern, and the "no frames found" error message to all use frame-NNNNNN.png consistently
• Fixed the unit test mock (was returning frame_0001.png filenames, masking the bug) and added a regression test that confirms the old underscore format is correctly rejected

Resolves #4774

Changes

• electron/ipc/handlers/demo.ts: updated framePattern regex to /^frame-\d{6}\.png$/, updated ffmpeg -i input to frame-%06d.png, updated error message text
• electron/ipc/handlers/__tests__/demo.handlers.test.ts: corrected mock filenames to frame-000001.png format, added assertion on ffmpeg -i argument, added regression test verifying old frame_NNNN.png names throw

Testing

Unit tests pass with the corrected mocks. The regression test covers the previously-broken path and confirms the old format is rejected as expected.

[gregpriday]

Fixes regression from #4785. Frame naming convention changed in capture but not in encoder.

Regression audit for training data.