feat(themes): executive-slate app launcher becomes the boreal-style icon grid

New themes/executive-slate/launcher.rasi: @import the sibling rofi.rasi
whole-swap, reshape drun into the 4x3 horizontal-flow grid of 56px icons
(mirrors boreal's launcher), cards at the theme's 4px radius so the flat
control-panel identity holds; the selected card keeps the accent-edge
highlight. Text menus are untouched. Picked up automatically by
hasLauncherOverride in rofi.nix.

V2: headless VM capture (before: vertical list; after: grid renders,
filter/arrow/backspace drive it correctly). Boreal grid re-captured, no
regression. Guest rendering caveat per docs/TESTING.md section 5 applies
to polish judgments; layout and highlight behavior are the claims here.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

themes/executive-slate/launcher.rasi

@@ -0,0 +1,44 @@
+/**
+ * Executive Slate — app-launcher override (themes/<slug>/launcher.rasi).
+ * Installed to ~/.local/share/rofi/themes/launcher.rasi and used only by
+ * `rofi -show drun` (the SUPER+Space / SUPER+D launcher). It inherits the
+ * flat slate panel from the sibling rofi.rasi (this whole-swap theme,
+ * which HM deploys under its source basename — rofi.rasi — not custom.rasi),
+ * then reshapes the list into a 4×3 grid of large application icons.
+ * Text menus keep the list — they don't use this theme. Cards stay at the
+ * theme's 4px radius (sharp "control panel" identity), and the selected
+ * card keeps the one-accent-edge highlight from rofi.rasi.
+ */
+
+@import "rofi.rasi"
+
+window { width: 780px; }
+
+listview {
+    columns: 4;
+    lines: 3;
+    fixed-columns: true;
+    spacing: 10px;
+    padding: 6px 0px 0px 0px;
+    /* Fill row-by-row so Down at a column's foot pages to the next screen
+       instead of jumping to the top of the next column (rofi's default
+       Vertical flow). Mirrors the theme-picker grid in rofi.nix. */
+    flow: horizontal;
+}
+
+element {
+    orientation: vertical;
+    padding: 16px 8px;
+    spacing: 10px;
+    border-radius: 4px;
+}
+
+element-icon {
+    size: 56px;
+    horizontal-align: 0.5;
+}
+
+element-text {
+    horizontal-align: 0.5;
+    vertical-align: 0.5;
+}