feat(native): make the topbar the window's title bar

[omartelo]

The native window opened under the system's title bar, a second strip over the topbar (on GNOME, Gio's own light bar, blind to the theme). The topbar is now the title bar. Mockup: https://claude.ai/artifact/1TNrcGD7UVntLeL5CN4kNP

• The window opens with Decorated(false). The empty run of the topbar after the tabs moves the window; a double click maximizes or restores.
• Minimize, maximize and close sit right of Settings, drawn in each desktop's look: Windows 11's 46dp corner buttons, GNOME's round buttons on the side and in the order of button-layout (read from the settings portal), Breeze's glyphs when XDG_CURRENT_DESKTOP is KDE.
• macOS keeps its own traffic lights; the topbar leaves them 78dp at the left and the lich mark follows them.
• A desktop that decorates its windows itself keeps its bar and lich draws no buttons: a Wayland compositor answering server-side to the client-side request, or an X11 tiling WM (by _NET_WM_NAME).
• Gio fork bumped to lichdotdev/gio c0397443 (branch drop-event): Wayland asks xdg-decoration for the mode the app wants; X11 sets _MOTIF_WM_HINTS, moves and resizes through _NET_WM_MOVERESIZE and keeps the WM's decoration under a tiling WM; both turn a double click on a move area into maximize.
• Tooltips in all seven locales; docs/ceilings.md names what is out (Snap Layouts, KDE decoration themes, X11 corners and shadow, macOS light position and double click).

Verification

• native/: gofmt, go vet ./..., go test ./... clean; fork: new caption tests pass.
• Rig: KWin --virtual with an isolated backend.
  • Wayland: lich asked client-side and KWin granted it (set_mode(1) then configure(1)); KDE and GNOME buttons drawn (-shot).
  • X11 (Xwayland managed by KWin): _MOTIF_WM_HINTS = 0x2, 0x0, ..., WM_CLASS lich, no frame, GNOME buttons drawn.
• Not verified: dragging, edge resize and the double click by hand (the rig has no input); what Hyprland, sway and niri answer; Windows and macOS were not run.