feat: inline preview respects window side (left/right)

[adriancmiranda]

Closes #121

Note

Float-based previewers (markdown, archive, cert, binary, font, key, sqlite) do not yet respect window. Full support, including per-extension override and size, is deferred to after #92 and the size key are implemented.

Summary

• Propagates window config through the full call chain: init.lua -> image.lua -> preview_route.lua -> inline.lua
• Inline preview() now accepts opts.window ('left'/'right') and uses it to decide the split direction on first open
• Existing marked preview windows are always reused on their current side, regardless of window value -- preserves the Oil.nvim workflow where the preview stays in the same split
• Adds window = nil to the default config schema so cfg.window is always a valid field
• When Oil calls M.preview() without opts, cfg.window is now consulted as fallback
• Video preview (video.lua) now respects window for both thumbnail and animated paths; Oil passes cfg.window explicitly when calling video.preview()
• Fixes model.lua unwrap guard: source_win.window is only used as a window ID when it is actually a number; string position values ('left', 'right') are ignored, preventing E5108 regression introduced by this branch

Test plan

• Set window = 'left' in setup: first <leader>p in Oil opens image preview to the left of Oil
• Set window = 'left' in setup: first <leader>p in Oil opens video preview to the left of Oil
• Subsequent <leader>p presses reuse the same split, regardless of which file is previewed
• :GlimpsePreview left and :GlimpsePreview right still work from any buffer
• No window in config: behavior unchanged (preview opens to the right)
• Navigate to a 3D model file in Oil and trigger preview: no E5108 error