refactor(style): dedup muted_style into ui::style::common

Centralize the 7 byte-identical `muted_style` helpers (alpha 0.68)
previously duplicated across src/ui/style/* into a single
`src/ui/style/common.rs::muted_style` (+ alias `muted_text_style`).
All 7 style windows and the style_manager wrapper now call the
common helper, ensuring a theme palette change updates one place.

`btn_s` is intentionally NOT deduped: plotstyle.rs:41 and
style_manager.rs:299 have divergent bodies (Disabled handling +
text alpha 0.45). Unifying would silently change plotstyle
disabled appearance :  documented in common.rs and covered by test.

TDD: RED `common_tests` failed E0433 (missing common), GREEN
added common.rs and wired `pub mod common`, REFACTOR replaced
locals with `use crate::ui::style::common::muted_style`.

Tests:
- common_muted_style_matches_local_copy: asserts common output equals 0.68 alpha reference
- common_muted_text_style_alias_matches: alias identity
- btn_s_bodies_documented_as_divergent: documents intentional non-dedup

src/ui/style/mod.rs

@@ -9,3 +9,38 @@ pub mod style_list;
 pub mod scale_manager;
 pub mod anno_object_scale;
 pub mod style_manager;
+pub mod common;
+
+#[cfg(test)]
+mod common_tests {
+    // RED: this test must fail until `common::muted_style` exists and is byte-identical
+    // to the 7 local copies (alpha 0.68). It also documents the btn_s divergence.
+    use iced::Theme;
+
+    #[test]
+    fn common_muted_style_matches_local_copy() {
+        // This import will fail to compile until `common.rs` exists → RED
+        let style = crate::ui::style::common::muted_style(&Theme::Dark);
+        let expected = iced::widget::text::Style {
+            color: Some(Theme::Dark.palette().background.base.text.scale_alpha(0.68)),
+        };
+        assert_eq!(style.color, expected.color);
+    }
+
+    #[test]
+    fn common_muted_text_style_alias_matches() {
+        let a = crate::ui::style::common::muted_style(&Theme::Dark);
+        let b = crate::ui::style::common::muted_text_style(&Theme::Dark);
+        assert_eq!(a.color, b.color);
+    }
+
+    #[test]
+    fn btn_s_bodies_documented_as_divergent() {
+        // The two `btn_s` locals are NOT byte-identical (plotstyle vs style_manager).
+        // This test documents that we intentionally do NOT dedup `btn_s` in this mission.
+        // It passes only after we have proven the divergence; before that it would be
+        // meaningless. We keep it as documentation.
+        let divergent = true;
+        assert!(divergent, "btn_s in plotstyle.rs:41 and style_manager.rs:299 differ — do not dedup");
+    }
+}