fix(dfm,theme): silence GTK markup + Adwaita dark-theme warnings

[VictorGSchneider]

Follow-up to #95. Two more warnings observed when launching dfm:

Adwaita-WARNING: Using GtkSettings:gtk-application-prefer-dark-theme with libadwaita is unsupported. Please use AdwStyleManager:color-scheme instead.
Gtk-WARNING: Failed to set text 'Notes & Tags' from markup due to error parsing markup: Entity did not end with a semicolon...
Gtk-WARNING: Failed to set text 'Caution: this setting can cause issues with multiline prompts in zsh < 5.7.1 (see #5765)' from markup due to error parsing markup...

Commits

fix(dfm): escape Pango markup in row titles from config comments

Adw.PreferencesRow titles and subtitles are parsed as Pango markup, so config keys/comments containing & or < (e.g. the oh-my-zsh .zshrc comment "Caution: ... in zsh < 5.7.1") triggered repeated Failed to set text from markup Gtk warnings on every row redraw.

• Run field.key and field.comment through GLib.markup_escape_text before passing them to Adw.SwitchRow / Adw.ActionRow in window_config_page_fields.py. Single escape point in create_field_row covers every field type (toggle, slider, color, number, path, keybind, font, text).
• Fix the literal 'Notes & Tags' group title to 'Notes & Tags' in window_config_page_sections.py.

fix(theme): drop deprecated prefer-dark-theme from gtk-4.0 settings

libadwaita ignores gtk-application-prefer-dark-theme and prints Adwaita-WARNING: Using GtkSettings:gtk-application-prefer-dark-theme with libadwaita is unsupported on every GTK4 app launch.

Dark mode for GTK4 / libadwaita is already driven by the gsettings key org.gnome.desktop.interface color-scheme = 'prefer-dark', which theme/pacman-hooks/stoa-theme-enforce sets (line 44). So removing the deprecated line is a no-op for visual behaviour and silences the warning.

Kept in theme/gtk-3.0/settings.ini — the key is still valid for GTK3.

Test plan

• python3 -m py_compile on the touched DFM modules.
• Inspected the diff against current main (post-fix(dfm): handle non-directory entries in ~/.config scan #95 merge).
• Manual: launch dfm after git pull — the three warnings above should be gone. (Run sudo /usr/local/bin/stoa-theme-enforce once if gsettings get org.gnome.desktop.interface color-scheme doesn't already return 'prefer-dark'.)

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