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/common.rs

@@ -0,0 +1,28 @@
+//! Style helpers shared across the style windows.
+//!
+//! Centralizes `muted_style` (and its alias `muted_text_style`) which were
+//! previously duplicated in 7 style files with byte-identical bodies (alpha
+//! 0.68). A single source ensures a theme change updates one place.
+//!
+//! **`btn_s` is intentionally NOT deduped here.** The two `btn_s` locals
+//! (`plotstyle.rs:41` and `style_manager.rs:299`) have divergent bodies:
+//! `style_manager` handles `Status::Disabled` (text alpha 0.45) while
+//! `plotstyle` does not. Unifying them would silently change the plotstyle
+//! disabled appearance. See `cargo test --lib ui::style::common_tests`.
+
+use iced::Theme;
+
+/// Muted secondary text style — `background.base.text` at 68% opacity.
+pub(crate) fn muted_style(theme: &Theme) -> iced::widget::text::Style {
+    iced::widget::text::Style {
+        color: Some(theme.palette().background.base.text.scale_alpha(0.68)),
+    }
+}
+
+/// Alias for call sites that used `muted_text_style` (e.g. `style_manager`).
+/// Same implementation as [`muted_style`]; provided so existing imports
+/// can be unified without renaming at the call site.
+#[allow(dead_code)]
+pub(crate) fn muted_text_style(theme: &Theme) -> iced::widget::text::Style {
+    muted_style(theme)
+}