feat(converter): add --keep-filenames flag to preserve original page filenames

Add a new `--keep-filenames` boolean flag (no shorthand, default false) on
both `optimize` and `watch` commands. When enabled, page entries in the
output CBZ preserve their original base filenames instead of being
renumbered to `%04d` sequential indices. Extension is swapped on format
conversion (e.g. `001.png` → `001.webp`), split pages get `-NN` suffixes
on the original stem, and duplicate base names fall back to
`<stem>_<index>.<ext>`. Watch mode supports the flag via the
`keep-filenames` viper config key.

Design: data-driven `OriginalName` field on `manga.PageFile`, set at
extraction in `cbz_loader.ExtractChapter`. The `Converter` interface is
unchanged. Flag off → behavior identical to before.

Closes #216

internal/manga/page.go

@@ -13,4 +13,10 @@ type PageFile struct {
 	IsSplitted bool
 	// SplitPartIndex is the part index when the page was split.
 	SplitPartIndex uint16
+	// OriginalName is the base filename (e.g. "page01.png") of the page as
+	// it appeared in the source archive, recorded when the --keep-filenames
+	// flag is enabled. Empty when the flag is off or the source name is
+	// unknown. When set, downstream code uses this stem (with the final
+	// Extension swapped in) instead of the default %04d sequential name.
+	OriginalName string
 }