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/status_bar.rs

@@ -0,0 +1,109 @@
+use crate::theme::Theme;
+use agent_core::SessionStats;
+use permissions::Mode;
+use ratatui::{
+    layout::Rect,
+    text::{Line, Span},
+    widgets::Paragraph,
+    Frame,
+};
+
+pub struct StatusBar {
+    model: String,
+    mode: Mode,
+    stats: SessionStats,
+    last_tool: Option<String>,
+    theme: Theme,
+}
+
+impl StatusBar {
+    pub fn new(model: String, mode: Mode, theme: Theme) -> Self {
+        Self {
+            model,
+            mode,
+            stats: SessionStats::new(),
+            last_tool: None,
+            theme,
+        }
+    }
+
+    pub fn update_stats(&mut self, stats: SessionStats) {
+        self.stats = stats;
+    }
+
+    pub fn set_last_tool(&mut self, tool: String) {
+        self.last_tool = Some(tool);
+    }
+
+    pub fn render(&self, frame: &mut Frame, area: Rect) {
+        let elapsed = self.stats.start_time.elapsed().unwrap_or_default();
+        let elapsed_str = SessionStats::format_duration(elapsed);
+
+        let (mode_str, mode_icon) = match self.mode {
+            Mode::Plan => ("Plan", "🔍"),
+            Mode::AcceptEdits => ("AcceptEdits", "✏️"),
+            Mode::Code => ("Code", "⚡"),
+        };
+
+        let last_tool_str = self
+            .last_tool
+            .as_ref()
+            .map(|t| format!("● {}", t))
+            .unwrap_or_else(|| "○ idle".to_string());
+
+        // Build status line with colorful sections
+        let separator_style = self.theme.status_bar;
+        let mut spans = vec![
+            Span::styled(" ", separator_style),
+            Span::styled(mode_icon, self.theme.status_bar),
+            Span::styled(" ", separator_style),
+            Span::styled(mode_str, self.theme.status_bar),
+            Span::styled(" │ ", separator_style),
+            Span::styled("⚙", self.theme.status_bar),
+            Span::styled(" ", separator_style),
+            Span::styled(&self.model, self.theme.status_bar),
+            Span::styled(" │ ", separator_style),
+            Span::styled(
+                format!("{} msgs", self.stats.total_messages),
+                self.theme.status_bar,
+            ),
+            Span::styled(" │ ", separator_style),
+            Span::styled(
+                format!("{} tools", self.stats.total_tool_calls),
+                self.theme.status_bar,
+            ),
+            Span::styled(" │ ", separator_style),
+            Span::styled(
+                format!("~{} tok", self.stats.estimated_tokens),
+                self.theme.status_bar,
+            ),
+            Span::styled(" │ ", separator_style),
+            Span::styled("⏱", self.theme.status_bar),
+            Span::styled(" ", separator_style),
+            Span::styled(elapsed_str, self.theme.status_bar),
+            Span::styled(" │ ", separator_style),
+            Span::styled(last_tool_str, self.theme.status_bar),
+        ];
+
+        // Add help text on the right
+        let help_text = " ? /help ";
+
+        // Calculate current length
+        let current_len: usize = spans.iter()
+            .map(|s| unicode_width::UnicodeWidthStr::width(s.content.as_ref()))
+            .sum();
+
+        // Add padding
+        let padding = area
+            .width
+            .saturating_sub((current_len + help_text.len()) as u16);
+
+        spans.push(Span::styled(" ".repeat(padding as usize), separator_style));
+        spans.push(Span::styled(help_text, self.theme.status_bar));
+
+        let line = Line::from(spans);
+        let paragraph = Paragraph::new(line);
+
+        frame.render_widget(paragraph, area);
+    }
+}