ui: force alignment of modal callback function

fullscreen modal dialog was added in 4297c139. the callback function to
that modal *must* be aligned according to the target architecture.
failing to do so may result in "incorrect alignment" panic.

this somehow worked in 4297c139 but sometimes panic. i suspect this may
be due to PIE-enabled build, possibly in conjuction with some zig v0.10
compiler bugs.

ref #20
ref #5

src/ui/poweroff.zig

@@ -31,7 +31,7 @@ export fn nm_poweroff_btn_callback(_: *lvgl.LvEvent) void {
 }
 
 /// poweroff confirmation screen callback.
-fn poweroffModalCallback(btn_idx: usize) void {
+fn poweroffModalCallback(btn_idx: usize) align(@alignOf(widget.ModalButtonCallbackFn)) void {
     // proceed = 0, cancel = 1
     if (btn_idx != 0) {
         return;

src/ui/widget.zig

@@ -50,10 +50,12 @@ pub const ModalButtonCallbackFn = *const fn (index: usize) void;
 /// while all heap-alloc'ed resources are free'd automatically right before cb is called,
 /// the value of title, text and btns args must live at least as long as cb; they are
 /// memory-managed by the callers.
+///
+/// note: the cb callback must have @alignOf(ModalbuttonCallbackFn) alignment.
 pub fn modal(title: [*:0]const u8, text: [*:0]const u8, btns: []const [*:0]const u8, cb: ModalButtonCallbackFn) !void {
     const win = try lvgl.createWindow(null, 60, title);
     errdefer win.winobj.destroy(); // also deletes all children created below
-    win.winobj.setUserdata(@ptrCast(?*const anyopaque, cb));
+    win.winobj.setUserdata(cb);
 
     const wincont = win.content();
     wincont.flexFlow(.column);