fix: correct video integration after comic generation

PromptForGalleryVideos now returns []string paths (from recursive walk)
instead of []int page numbers, so GenerateSelectedVideos always has the
exact path to each gallery PNG regardless of which comics/<slug>/
subdirectory it lives in.

Previously, GenerateSelectedVideos passed "." as galleryPath and called
loadGalleryImage with a non-recursive filepath.Glob that could not find
PNGs in subdirectories, causing video generation to always fail with
"no gallery image found".

Additional changes:
- Add VeoGenerator.GenerateVideoFromPath that accepts a full image path
  and writes the MP4 next to the source PNG
- Make runStoryVideos non-fatal: video errors print a warning and return
  nil so comic/PDF/narration outputs are never invalidated by Veo errors
- Add filterPathsByPages helper and tests for the new behaviour
- Add TestFindGalleryPages_Recursive to cover the comics/<slug>/ layout

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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