Replace platform-dependent info glyph in AI prefs model card with cairo-drawn icon

[andriiryzhkov]

The info icon next to each model in the AI preferences was a unicode glyph (ⓘ, U+24D8). How it renders depends on the system's font fallback chain – it looked different on macOS, Windows, and across Linux distros, sometimes a tiny circled "i", sometimes a fat blob. Bad UX for an icon that's supposed to invite clicking.

Replaced it with a cairo-drawn icon, same approach dtgtk_button uses for its own icons – the glyph is now identical everywhere darktable runs.

The cell-renderer side of this is reusable: extracted as dtgtk_paint_cell under src/dtgtk/, so any future "draw a cairo paint in a tree column" need (help icons elsewhere, status indicators, etc.) is a one-line dtgtk_paint_cell_new(paint_fn, 0, NULL) call away.

Two new bits worth flagging:

• dtgtk_cairo_paint_info in src/dtgtk/paint.c – the "i in a circle" glyph itself. Stroked at the same line weight (line_scaling=1) as the rest of DT's icons, so it sits visually alongside button icons.
• dtgtk_paint_cell in src/dtgtk/paint_cell.{h,c} – generic cell renderer wrapping a DTGTKCairoPaintIconFunc. Mirrors dtgtk_button in shape (public struct fields, single constructor).

[TurboGit]

Thanks!