feat: GNOME-style welcome, terminal launcher config, and UX polish

[leandromqrs]

Welcome page (GNOME Initial Setup style):

• edge-to-edge banner (no margins or rounded frame) and a greeting carousel cycling "Welcome" through the pinned languages every 5 s, filtered by font coverage, mirroring GisWelcomeWidget

Terminal launcher:

• new [terminal] section in sirius.toml (command, show_button); the header-bar button is hidden by default and Ctrl+Shift+P always opens the configured terminal

Time zone page:

• drop the libgweather dependency; the chooser is backed only by the tzdb tables, like GNOME Initial Setup
• the selected city shows as a permanent label above the pin (name, zone, UTC offset, local time) instead of a hover tooltip

Language & keyboard pages:

• selection now uses the default Adwaita selected-row background instead of a check mark; the "More…" expander row is gone (the list scrolls)

Network page:

• rows connect on activation (no oversized Connect button); rescan is a flat refresh icon pinned to the group header, outside the scroll area so the scrollbar never covers it

Storage page:

• disk picker rows use real grouped GtkCheckButton radios instead of a custom check mark

Chooser lists (language, keyboard, network):

• suppress the GTK undershoot shadow that grayed the card's bottom edge and corners whenever the list could scroll

i18n:

• set_ui_language now also setlocale()s the chosen locale: on the live ISO the process starts in the C locale (empty service environment), where glibc gettext ignores LANGUAGE entirely
• language page logo capped with GtkImage pixel_size instead of a GtkPicture that let the intrinsic PNG size win

Tests:

• interactive GTK tests now run on a single GTK-owning thread (GTK can only be initialized from one thread); new assertions for arrow centering, welcome banner/greetings, pin label, C-locale language switch, and radio-less timezone search