OCR language selector: show download size before fetching

[Kayforkind]

Switching OCR languages downloads that language's traineddata. Show the approximate size next to each language in the selector so users on slow connections know what to expect.

Scope: UI text addition; sizes can be hardcoded from the files in public/tess/.

[Kayforkind]

Done — the OCR language selector now shows each language's one-time download size: English · 2.8 MB, Español · 1.1 MB, Français · 0.6 MB, Deutsch · 0.8 MB (from the traineddata.gz files under public/tessdata/).