Initial implementation of Improvise TUI

Multi-dimensional data modeling terminal application with:
- Core data model: categories, items, groups, sparse cell store
- Formula system: recursive-descent parser, named formulas, WHERE clauses
- View system: Row/Column/Page axes, tile-based pivot, page slicing
- JSON import wizard (interactive TUI + headless auto-mode)
- Command layer: all mutations via typed Command enum for headless replay
- TUI: Ratatui grid, tile bar, formula/category/view panels, help overlay
- Persistence: .improv (JSON), .improv.gz (gzip), CSV export, autosave
- Static binary via x86_64-unknown-linux-musl + nix flake devShell
- Headless mode: --cmd '{"op":"..."}' and --script file.jsonl

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui/view_panel.rs

@@ -0,0 +1,62 @@
+use ratatui::{
+    buffer::Buffer,
+    layout::Rect,
+    style::{Color, Modifier, Style},
+    widgets::{Block, Borders, Widget},
+};
+
+use crate::model::Model;
+use crate::ui::app::AppMode;
+
+pub struct ViewPanel<'a> {
+    pub model: &'a Model,
+    pub mode: &'a AppMode,
+    pub cursor: usize,
+}
+
+impl<'a> ViewPanel<'a> {
+    pub fn new(model: &'a Model, mode: &'a AppMode, cursor: usize) -> Self {
+        Self { model, mode, cursor }
+    }
+}
+
+impl<'a> Widget for ViewPanel<'a> {
+    fn render(self, area: Rect, buf: &mut Buffer) {
+        let is_active = matches!(self.mode, AppMode::ViewPanel);
+        let border_style = if is_active {
+            Style::default().fg(Color::Blue)
+        } else {
+            Style::default().fg(Color::DarkGray)
+        };
+
+        let block = Block::default()
+            .borders(Borders::ALL)
+            .border_style(border_style)
+            .title(" Views [Enter] switch [n]ew [d]elete ");
+        let inner = block.inner(area);
+        block.render(area, buf);
+
+        let view_names: Vec<&str> = self.model.views.keys().map(|s| s.as_str()).collect();
+        let active = &self.model.active_view;
+
+        for (i, view_name) in view_names.iter().enumerate() {
+            if inner.y + i as u16 >= inner.y + inner.height { break; }
+
+            let is_selected = i == self.cursor && is_active;
+            let is_active_view = *view_name == active.as_str();
+
+            let style = if is_selected {
+                Style::default().fg(Color::Black).bg(Color::Blue).add_modifier(Modifier::BOLD)
+            } else if is_active_view {
+                Style::default().fg(Color::Blue).add_modifier(Modifier::BOLD)
+            } else {
+                Style::default()
+            };
+
+            let prefix = if is_active_view { "▶ " } else { "  " };
+            buf.set_string(inner.x, inner.y + i as u16,
+                format!("{prefix}{view_name}"),
+                style);
+        }
+    }
+}