TUI walking skeleton (Phase 4)

First implementation milestone: Cargo project, dependencies,
and a minimal but functional TUI shell built on Ratatui +
Crossterm + Tokio in the Elm-style update/view pattern
(Candidate A from Phase 2/3 selection).

Includes:
- Three-region layout: items list (left), output + input + hint
  (right), bottom status bar with mode-aware shortcuts.
- Two themes (light, dark) plus COLORFGBG auto-detect, per
  NFR-7. CLI: --theme {light,dark}, --log-file <path>.
- Input modes per ADR-0003: simple (default), advanced, with
  the `:` one-shot escape including immediate prompt reaction
  ("Advanced:" label, advanced border) and auto-inserted space
  after a leading `:` in simple mode.
- App-level commands: `quit`/`q`, `mode simple`/`mode advanced`
  (canonical list per ADR-0003 — remaining commands land in
  later iterations).
- File logging via tracing, defaulting to ~/.rdbms-playground/
  playground.log so the TUI is not corrupted by stdio.

Testing per ADR-0008:
- Tier 1: 29 unit tests covering input handling, mode switch,
  one-shot escape, auto-space, output buffering, CLI parsing.
- Tier 2: 4 insta snapshots (default simple/advanced/light,
  one-shot active) of TestBackend frames.
- Tier 3: 7 integration tests driving synthetic events through
  App::update + render path.

All green: 36 tests, 0 failures, 0 skips. Clippy clean with
nursery lints enabled.

src/theme.rs

@@ -0,0 +1,69 @@
+//! Theme and colour palette.
+//!
+//! Two themes are provided — one for light terminal backgrounds
+//! and one for dark — per NFR-7. The palette is intentionally
+//! small for the walking skeleton; it grows as more views are
+//! added. Contrast is chosen against the target background so
+//! that foreground text meets WCAG-AA (NFR-5) on both variants.
+
+use ratatui::style::Color;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Background {
+    Light,
+    Dark,
+}
+
+#[derive(Debug, Clone)]
+pub struct Theme {
+    pub background: Background,
+    pub bg: Color,
+    pub fg: Color,
+    pub muted: Color,
+    pub border: Color,
+    pub border_advanced: Color,
+    pub mode_simple: Color,
+    pub mode_advanced: Color,
+    pub system: Color,
+    pub error: Color,
+}
+
+impl Theme {
+    #[must_use]
+    pub const fn dark() -> Self {
+        Self {
+            background: Background::Dark,
+            bg: Color::Rgb(0x18, 0x1B, 0x22),
+            fg: Color::Rgb(0xE6, 0xE6, 0xE6),
+            muted: Color::Rgb(0x8B, 0x90, 0x9A),
+            border: Color::Rgb(0x4A, 0x52, 0x65),
+            border_advanced: Color::Rgb(0xE0, 0x60, 0x60),
+            mode_simple: Color::Rgb(0x6E, 0xC4, 0xFF),
+            mode_advanced: Color::Rgb(0xFF, 0x9E, 0x6B),
+            system: Color::Rgb(0x9F, 0xD8, 0x91),
+            error: Color::Rgb(0xFF, 0x6B, 0x6B),
+        }
+    }
+
+    #[must_use]
+    pub const fn light() -> Self {
+        Self {
+            background: Background::Light,
+            bg: Color::Rgb(0xFA, 0xFA, 0xF7),
+            fg: Color::Rgb(0x1A, 0x1F, 0x2C),
+            muted: Color::Rgb(0x60, 0x66, 0x73),
+            border: Color::Rgb(0xB6, 0xBC, 0xC8),
+            border_advanced: Color::Rgb(0xC2, 0x3A, 0x3A),
+            mode_simple: Color::Rgb(0x21, 0x69, 0xC7),
+            mode_advanced: Color::Rgb(0xB0, 0x4A, 0x12),
+            system: Color::Rgb(0x2E, 0x7C, 0x3C),
+            error: Color::Rgb(0xC0, 0x39, 0x2B),
+        }
+    }
+}
+
+impl Default for Theme {
+    fn default() -> Self {
+        Self::dark()
+    }
+}