feat(tui): implement live theme preview system

- Add preview_theme field to AppState for temporary theme previews
- Add set_preview_theme, get_effective_theme_colors, start_theme_preview,
  cancel_theme_preview, and confirm_theme_preview methods
- Add ModalResult::ActionContinue for live preview without closing modal
- Add ModalAction variants: PreviewTheme, RevertTheme, ConfirmTheme
- Update ThemeSelectorModal to emit preview actions on navigation
- Add color swatch display in theme selector rows
- Handle new theme actions in event loop
- Update ThemePicker to use ThemeSelectorModal instead of interactive builder
- Add comprehensive tests for new preview functionality

Author: echobt

src/cortex-tui/src/app/methods.rs

@@ -631,6 +631,43 @@ impl AppState {
     }
 }
 
+// ============================================================================
+// APPSTATE METHODS - Theme Preview
+// ============================================================================
+
+impl AppState {
+    /// Start previewing a theme.
+    ///
+    /// This updates the cached theme colors to show the preview theme,
+    /// without changing the active theme.
+    pub fn start_theme_preview(&mut self, theme_name: &str) {
+        self.set_preview_theme(Some(theme_name.to_string()));
+    }
+
+    /// Cancel theme preview and revert to the original (active) theme.
+    ///
+    /// Restores the cached colors to the active theme.
+    pub fn cancel_theme_preview(&mut self) {
+        self.set_preview_theme(None);
+    }
+
+    /// Confirm the previewed theme as the active theme.
+    ///
+    /// Makes the preview theme the new active theme and clears the preview state.
+    pub fn confirm_theme_preview(&mut self) {
+        if let Some(preview) = self.preview_theme.take() {
+            self.active_theme = preview.clone();
+            // Colors are already set to the preview theme, just need to clear preview state
+            self.preview_theme = None;
+        }
+    }
+
+    /// Check if a theme preview is active.
+    pub fn has_theme_preview(&self) -> bool {
+        self.preview_theme.is_some()
+    }
+}
+
 // ============================================================================
 // APPSTATE METHODS - Operation Mode
 // ============================================================================

src/cortex-tui/src/app/state.rs

@@ -106,6 +106,8 @@ pub struct AppState {
     pub input_mode: crate::interactive::InputMode,
     /// Active theme name
     pub active_theme: String,
+    /// Preview theme name (for live theme preview in selector)
+    pub preview_theme: Option<String>,
     /// Cached theme colors for quick access
     pub theme_colors: ThemeColors,
     /// Cached markdown theme for quick access
@@ -238,6 +240,7 @@ impl AppState {
             diff_scroll: 0,
             input_mode: crate::interactive::InputMode::Normal,
             active_theme: "dark".to_string(),
+            preview_theme: None,
             theme_colors: ThemeColors::dark(),
             markdown_theme: MarkdownTheme::default(),
             compact_mode: false,
@@ -372,10 +375,32 @@ impl AppState {
     /// Change the active theme
     pub fn set_theme(&mut self, name: &str) {
         self.active_theme = name.to_string();
+        self.preview_theme = None; // Clear any preview when setting the theme
         self.theme_colors = ThemeColors::from_name(name);
         self.markdown_theme = MarkdownTheme::from_name(name);
     }
 
+    /// Set a preview theme for live preview functionality
+    ///
+    /// Updates the cached theme_colors to the preview theme colors.
+    pub fn set_preview_theme(&mut self, theme: Option<String>) {
+        self.preview_theme = theme.clone();
+        // Update cached colors based on preview or active theme
+        let effective_theme = theme.as_deref().unwrap_or(&self.active_theme);
+        self.theme_colors = ThemeColors::from_name(effective_theme);
+        self.markdown_theme = MarkdownTheme::from_name(effective_theme);
+    }
+
+    /// Get the effective theme colors (preview if set, otherwise active)
+    pub fn get_effective_theme_colors(&self) -> &ThemeColors {
+        &self.theme_colors
+    }
+
+    /// Get the name of the effective theme (preview if set, otherwise active)
+    pub fn get_effective_theme_name(&self) -> &str {
+        self.preview_theme.as_deref().unwrap_or(&self.active_theme)
+    }
+
     /// Get AdaptiveColors from the current theme
     pub fn adaptive_colors(&self) -> crate::ui::AdaptiveColors {
         crate::ui::AdaptiveColors::from_theme_colors(&self.theme_colors)

src/cortex-tui/src/modal/mod.rs

@@ -97,6 +97,8 @@ pub enum ModalResult {
     Close,
     /// Perform an action and close
     Action(ModalAction),
+    /// Perform an action but keep the modal open (for live preview)
+    ActionContinue(ModalAction),
     /// Push a new modal on top of this one
     Push(Box<dyn Modal>),
     /// Replace this modal with another
@@ -172,6 +174,14 @@ pub enum ModalAction {
         api_key: String,
     },
 
+    // Theme Preview Actions
+    /// Preview a theme without applying it permanently
+    PreviewTheme(String),
+    /// Revert to the original theme (cancel preview)
+    RevertTheme,
+    /// Confirm and apply the previewed theme
+    ConfirmTheme(String),
+
     // Generic/Custom
     Custom(String),
 }
@@ -253,6 +263,10 @@ impl ModalStack {
                     self.pop();
                     ModalResult::Action(action)
                 }
+                ModalResult::ActionContinue(action) => {
+                    // Return the action but keep the modal open (for live preview)
+                    ModalResult::ActionContinue(action)
+                }
                 other => other,
             }
         } else {