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/style_manager.rs

@@ -324,9 +324,7 @@ fn btn_s(accent: bool) -> impl Fn(&Theme, Status) -> Style {
 }
 
 pub(crate) fn muted_text_style(theme: &Theme) -> iced::widget::text::Style {
-    iced::widget::text::Style {
-        color: Some(theme.palette().background.base.text.scale_alpha(0.68)),
-    }
+    crate::ui::style::common::muted_style(theme)
 }
 
 pub(crate) fn hdivider<'a>(width: iced::Length) -> Element<'a, Message> {