feat: per-place fonts + live theme preview in wbconf

[ebenali]

Adds Openbox-style per-place font configuration (you asked for font family/size selection) and makes wbconf fancier with a live, pixel-faithful theme preview.

Compositor — honour rc.xml fonts

waybox previously used a hardcoded sans 10pt everywhere. Now:

• Theme gains a FontSpec per place (active/inactive title, menu header, menu item, OSD); FontSpec.family is now std::string so parsed names are safely owned.
• The frame/menu/switcher style adapters set each resolved TextStyle.font from them — so a configured title font also drives the titlebar height (the metrics measure the label font).
• config.cpp parses the Openbox <theme><font place="..."><name>/<size>/<weight> block onto the loaded theme. New pure wb::font_place_from_name() maps the place names (incl. OSD aliases); unit-tested.

wbconf — fonts + preview (fancier)

• Fonts tab: a GtkFontDialogButton per place. RcDocument reads/writes the structured <font> elements as Pango "Family [Bold] Size" strings (round-trip unit-tested, incl. create/update/remove).
• Live Preview pane on the Theme tab: a GtkDrawingArea that renders sample active/inactive titlebars (with window buttons) and a menu using the same theme/style/render pipeline as the compositor, so it's pixel-faithful. It reflects the live, unsaved selection and redraws when the theme or any font changes. wbconf now links style/widget/render.
• data/rc.xml documents the <font> block.

Testing

• New unit tests: font_place_from_name, theme-font → resolved-style flow, and wbconf <font> round-trip.
• Full suite 27 green (g++/ASan/UBSan); clang + release clean.
• Headless: a 20pt title font visibly enlarges the titlebar; wbconf maps with the Fonts tab and the preview renders with no Cairo/Pango/GTK errors.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com