feat(gui): add a colour theme model with built-ins and file I/O

New gui::theme module: a Theme is a curated semantic palette (backgrounds,
text, accent, button, and the status colours) that serialises to TOML with
#rrggbb hex colours and builds an egui::Visuals. Missing fields fall back to
the built-in Default Dark via #[serde(default)], so partial/hand-trimmed
files still load. Three built-ins ship (Default Dark, Catppuccin Mocha,
Catppuccin Latte); user themes live as *.toml in ~/.config/pixelpass/themes/
and a user file overrides a built-in of the same name. Adds a `theme` field
to the GUI config (default "Default Dark"). Zero new deps (toml + a few
lines of hex parsing). 6 unit tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/common/config.rs

@@ -32,6 +32,10 @@ pub struct GuiSettings {
     /// text-only host screen.
     #[serde(default = "default_true")]
     pub show_qr: bool,
+    /// Name of the active GUI colour theme (a built-in, or a user file in
+    /// `~/.config/pixelpass/themes/`). Defaults to the built-in Default Dark.
+    #[serde(default = "default_theme")]
+    pub theme: String,
 }
 
 impl Default for GuiSettings {
@@ -39,6 +43,7 @@ impl Default for GuiSettings {
         Self {
             close_to_tray: false,
             show_qr: true,
+            theme: default_theme(),
         }
     }
 }
@@ -47,6 +52,10 @@ fn default_true() -> bool {
     true
 }
 
+fn default_theme() -> String {
+    "Default Dark".to_string()
+}
+
 /// Result of the first-run upstream measurement.
 ///
 /// `status = "unmeasured"` means we've never asked the user — show the