feat(ui): add TUI with streaming agent integration and theming

Add a new terminal UI crate (crates/app/ui) built with ratatui providing an
interactive chat interface with real-time LLM streaming and tool visualization.

Features:
- Chat panel with horizontal padding for improved readability
- Input box with cursor navigation and command history
- Status bar with session statistics and uniform background styling
- 7 theme presets: Tokyo Night (default), Dracula, Catppuccin, Nord,
  Synthwave, Rose Pine, and Midnight Ocean
- Theme switching via /theme <name> and /themes commands
- Streaming LLM responses that accumulate into single messages
- Real-time tool call visualization with success/error states
- Session tracking (messages, tokens, tool calls, duration)
- REPL commands: /help, /status, /cost, /checkpoint, /rewind, /clear, /exit

Integration:
- CLI automatically launches TUI mode when running interactively (no prompt)
- Falls back to legacy text REPL with --no-tui flag
- Uses existing agent loop with streaming support
- Supports all existing tools (read, write, edit, glob, grep, bash)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

crates/app/ui/src/components/input_box.rs

@@ -0,0 +1,144 @@
+use crate::theme::Theme;
+use crossterm::event::{KeyCode, KeyEvent};
+use ratatui::{
+    layout::Rect,
+    style::Style,
+    text::{Line, Span},
+    widgets::{Block, Borders, Padding, Paragraph},
+    Frame,
+};
+
+pub struct InputBox {
+    input: String,
+    cursor_position: usize,
+    history: Vec<String>,
+    history_index: usize,
+    theme: Theme,
+}
+
+impl InputBox {
+    pub fn new(theme: Theme) -> Self {
+        Self {
+            input: String::new(),
+            cursor_position: 0,
+            history: Vec::new(),
+            history_index: 0,
+            theme,
+        }
+    }
+
+    pub fn handle_key(&mut self, key: KeyEvent) -> Option<String> {
+        match key.code {
+            KeyCode::Enter => {
+                let message = self.input.clone();
+                if !message.trim().is_empty() {
+                    self.history.push(message.clone());
+                    self.history_index = self.history.len();
+                    self.input.clear();
+                    self.cursor_position = 0;
+                    return Some(message);
+                }
+            }
+            KeyCode::Char(c) => {
+                self.input.insert(self.cursor_position, c);
+                self.cursor_position += 1;
+            }
+            KeyCode::Backspace => {
+                if self.cursor_position > 0 {
+                    self.input.remove(self.cursor_position - 1);
+                    self.cursor_position -= 1;
+                }
+            }
+            KeyCode::Delete => {
+                if self.cursor_position < self.input.len() {
+                    self.input.remove(self.cursor_position);
+                }
+            }
+            KeyCode::Left => {
+                self.cursor_position = self.cursor_position.saturating_sub(1);
+            }
+            KeyCode::Right => {
+                if self.cursor_position < self.input.len() {
+                    self.cursor_position += 1;
+                }
+            }
+            KeyCode::Home => {
+                self.cursor_position = 0;
+            }
+            KeyCode::End => {
+                self.cursor_position = self.input.len();
+            }
+            KeyCode::Up => {
+                if !self.history.is_empty() && self.history_index > 0 {
+                    self.history_index -= 1;
+                    self.input = self.history[self.history_index].clone();
+                    self.cursor_position = self.input.len();
+                }
+            }
+            KeyCode::Down => {
+                if self.history_index < self.history.len() - 1 {
+                    self.history_index += 1;
+                    self.input = self.history[self.history_index].clone();
+                    self.cursor_position = self.input.len();
+                } else if self.history_index < self.history.len() {
+                    self.history_index = self.history.len();
+                    self.input.clear();
+                    self.cursor_position = 0;
+                }
+            }
+            _ => {}
+        }
+
+        None
+    }
+
+    pub fn render(&self, frame: &mut Frame, area: Rect) {
+        let is_empty = self.input.is_empty();
+
+        let block = Block::default()
+            .borders(Borders::ALL)
+            .border_style(self.theme.border_active)
+            .padding(Padding::horizontal(1))
+            .title(Line::from(vec![
+                Span::raw(" "),
+                Span::styled("✍", self.theme.border_active),
+                Span::raw(" "),
+                Span::styled("Input", self.theme.border_active),
+                Span::raw(" "),
+            ]));
+
+        // Display input with cursor
+        let (text_before, text_after) = if self.cursor_position < self.input.len() {
+            (
+                &self.input[..self.cursor_position],
+                &self.input[self.cursor_position..],
+            )
+        } else {
+            (&self.input[..], "")
+        };
+
+        let line = if is_empty {
+            Line::from(vec![
+                Span::styled("❯ ", self.theme.input_box_active),
+                Span::styled("▊", self.theme.input_box_active),
+                Span::styled(" Type a message...", Style::default().fg(self.theme.palette.fg_dim)),
+            ])
+        } else {
+            Line::from(vec![
+                Span::styled("❯ ", self.theme.input_box_active),
+                Span::styled(text_before, self.theme.input_box),
+                Span::styled("▊", self.theme.input_box_active),
+                Span::styled(text_after, self.theme.input_box),
+            ])
+        };
+
+        let paragraph = Paragraph::new(line).block(block);
+
+        frame.render_widget(paragraph, area);
+    }
+
+    pub fn clear(&mut self) {
+        self.input.clear();
+        self.cursor_position = 0;
+    }
+}