warn and auto-select weight when font_name has no Normal(400) face

[romanstingler]

ashell silently falls back to a different font when the configured font_name exists in the system font database but has no face with weight=Normal(400) (e.g. Terminus TTF, whose Regular face reports weight=500/Medium in its OS/2 table). This happens because cosmic-text's font matcher re-scores all faces and prefers the one whose weight is closest to the requested Normal(400), even if that means picking a font from a different family.

https://docs.rs/fontdb/0.23.0/src/fontdb/lib.rs.html#986
https://docs.rs/iced_core/0.14.0/src/iced_core/font.rs.html#74

Changes:
Add fontdb as a direct dependency (same version already used transitively by iced_layershell)(@MalpenZibo no idea i am really weighing if it is OK to keep it here or to put it into iced_layershell, but I think maybe you want to keep iced_layershell as slim as possible).

Add resolve_font() which:

• Finds all faces matching the configured font name.
• Picks the closest match to Normal style/stretch and weight 400.
• Builds an iced::Font with that face's actual weight and style, so cosmic-text's matcher stays on the right font.
• Warns clearly when no Normal(400) face is available.
• Warn when the font name isn't found at all (with a suggestion to use fc-list).

Fixes #768

[MalpenZibo]

Yep, it's something a little bit borderline and maybe could be placed in iced_layershell. I need to think about that