fix: unbreak the merge -- ComposeOptions grew at, the Explorer's literal didn't

`cargo build -p sylpheed-cli` after resolving the merge's one real conflict
(a rustfmt-only clash) was not "the merge is resolved" -- it only checked the
crate whose conflict I'd just looked at. `cargo check --workspace --all-targets`
finds the actual break: sylpheed-viewer's `compose_screen` builds a
`ComposeOptions` struct literal, and this branch added a field
(`at: Option<u32>`, for posing a composite mid-animation) that main's Explorer
code never had to name because every prior caller used `..Default::default()`
or the field didn't exist yet. Neither side's diff touched the other's line,
so nothing in the conflict pointed at it.

`at: None` is the only correct value here -- the doc comment on the field
already says so ("`None` keeps the settled composite, which is what every
existing caller wants"), and the Explorer's screen view has no notion of
mid-animation posing to wire up. This restores its exact prior behavior; nothing
about the Explorer's data source, static-only rule, or design changes.

PR #23 is red on this. Fixes it in place rather than opening a second PR, since
it is the same unit -- a merge I performed that did not actually leave a
building tree.

crates/sylpheed-viewer/src/iso_loader.rs

@@ -4453,6 +4453,7 @@ fn compose_screen(
             } else {
                 ComposeOptions::default().backdrop
             },
+            at: None,
         },
         Some(&visible),
     );