feat(gui): replace settings ⚙ placeholder with Bootstrap cog icon

Add iced_aw + iced_fonts (bootstrap feature) to load the Bootstrap Icons
font. The settings button and settings header now render the gear-fill
glyph (U+F3F8) via text("\u{F3F8}").font("bootstrap-icons") instead of
the raw ⚙ character which was not in iced's bundled Inter font.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -44,3 +44,5 @@ reqwest = { version = "0.12", features = ["blocking", "json"] }
 
 # GUI for the setup wizard
 iced = { version = "0.13", features = ["tokio"] }
+iced_aw = { version = "0.13", default-features = false }
+iced_fonts = { version = "0.3", features = ["bootstrap"] }