feat(tui): replace hard‑coded colors with Theme values and propagate Theme through UI rendering

- Introduce `Theme` import and pass a cloned `theme` instance to UI helpers (e.g., `render_editable_textarea`).
- Remove direct `Color` usage; UI now derives colors from `Theme` fields for placeholders, selections, ReAct components (thought, action, input, observation, final answer), status badges, operating mode badges, and model info panel.
- Extend `Theme` with new color fields for agent ReAct stages, badge foreground/background, and operating mode colors.
- Update rendering logic to apply these theme colors throughout the TUI (input panel, help text, status lines, model selection UI, etc.).
- Adjust imports to drop unused `Color` references.

crates/owlen-tui/src/chat_app.rs

@@ -9,7 +9,7 @@ use owlen_core::{
     types::{ChatParameters, ChatResponse, Conversation, ModelInfo, Role},
     ui::{AppState, AutoScroll, FocusedPanel, InputMode},
 };
-use ratatui::style::{Color, Modifier, Style};
+use ratatui::style::{Modifier, Style};
 use ratatui::text::{Line, Span};
 use textwrap::wrap;
 use tokio::{sync::mpsc, task::JoinHandle};
@@ -1372,9 +1372,10 @@ impl ChatApp {
                                         // Sync buffer to textarea before entering visual mode
                                         self.sync_buffer_to_textarea();
                                         // Set a visible selection style
-                                        self.textarea.set_selection_style(
-                                            Style::default().bg(Color::LightBlue).fg(Color::Black),
-                                        );
+                                        let selection_style = Style::default()
+                                            .bg(self.theme.selection_bg)
+                                            .fg(self.theme.selection_fg);
+                                        self.textarea.set_selection_style(selection_style);
                                         // Start visual selection at current cursor position
                                         self.textarea.start_selection();
                                         self.visual_start = Some(self.textarea.cursor());