74: Layout: in-place element editing + snap-to-grid + UX polish

[meduzapat]

Task-Url: #74

• Each tile carries a pencil button (visible while active) that opens the standard Element dialog with the device context preloaded.

• DialogDevice::editChild runs the device dialog's population pipeline silently (no run, no save) so the child dialog inherits a fresh, fully-wired state; the device dialog itself is never shown.

• Resolution of the device and element BoxButtons is a single linear scan of the navigator's devices collection — no observer changes, no back-pointers added to data.

• Tile icon split into a passive Gtk::Image (inactive) and a Gtk::ToggleButton (active). The toggle is the fire button; the light timer untoggles it on expiry, so rapid clicks can't bypass lightLocked.

• Pixbufs cached per element type — each icon is loaded from disk once.

• Press on a tile is consumed locally; clicks on the empty board deselect the current tile.

• DialogElement falls back to the main window as transient parent when the device dialog is hidden, keeping the dialog centered.

• New "Layout" page in Settings with a single grid-step scale (0–100, snaps to multiples of 10, default 0 = disabled).

• LayoutElement::onMotion applies a magnetic snap to the configured step; free movement when the user is more than step/3 from a gridline, snap once inside.

• New LayoutBoard (Gtk::Layout subclass) paints faint gridlines beneath the tiles and redraws live as the slider moves via a Settings::onLayoutGridChanged callback.

• The toggle-button styling for LED colors is now scoped with :not(.layout-element-icon) so the icon toggle gets its colored border/background from .layout-element-icon-fired.led-X, which covers all seven colors (R, G, B, Y, M, C, W). Previously Y/M/C/W fell through to default GTK styling and the toggle changed size on state changes.

address code review — hygiene and dead-code cleanup

• LayoutElement: fix onLightExpired indentation; remove dead onLayoutIdle(); static_cast the guaranteed Gtk::Layout parent in onMotion and drop the commented-out null guard; restore the informative iconForType comment; drop stray leading blank lines added to build()/fire()/onMotion().
• Layout::resolveButtons: document the trailing return as unreachable.
• style-base.css: revert comment-style churn (boxed-asterisk header and reflowed blocks) back to the house style used in theme.css; keep the functional active-tile and :not(.layout-element-icon) changes.
• Bring data/style.css in line with the house comment style used across data/ (file header, ── section dividers, em-dash minor notes); replace the boxed-asterisk Directory Navigator header. Comments only.