gui: add Wails v3 desktop app skeleton

- Go upgraded to 1.25.10 (required by Wails v3)
- Wails v3 installed (alpha.96)
- Frontend: Svelte+Vite scaffold in frontend/
- guimain.go: Wails GUI entry point (compiled with -tags gui)
- wails_service.go: stub service for Wails bindings
- Verstak desktop binary builds: go build -tags gui -o verstak-gui .
- CLI (./cmd/verstak/) unaffected
- Legacy HTTP GUI (internal/gui/) preserved as prototype
- Build: 24MB ELF binary with GTK4/WebKit2GTK-6

Build commands:
  CLI:  go build -o verstak ./cmd/verstak/
  GUI:  cd frontend && npm run build && go build -tags gui -o verstak-gui .

build/linux/nfpm/scripts/postinstall.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+
+# Update desktop database for .desktop file changes
+# This makes the application appear in application menus and registers its capabilities.
+if command -v update-desktop-database >/dev/null 2>&1; then
+  echo "Updating desktop database..."
+  update-desktop-database -q /usr/share/applications
+else
+  echo "Warning: update-desktop-database command not found. Desktop file may not be immediately recognized." >&2
+fi
+
+# Update MIME database for custom URL schemes (x-scheme-handler)
+# This ensures the system knows how to handle your custom protocols.
+if command -v update-mime-database >/dev/null 2>&1; then
+  echo "Updating MIME database..."
+  update-mime-database -n /usr/share/mime
+else
+  echo "Warning: update-mime-database command not found. Custom URL schemes may not be immediately recognized." >&2
+fi
+
+exit 0

build/linux/nfpm/scripts/postremove.sh

@@ -0,0 +1 @@
+#!/bin/bash

build/linux/nfpm/scripts/preinstall.sh

@@ -0,0 +1 @@
+#!/bin/bash

build/linux/nfpm/scripts/preremove.sh

@@ -0,0 +1 @@
+#!/bin/bash