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.

Cargo.toml

@@ -0,0 +1,35 @@
+[package]
+name = "rdbms-playground"
+version = "0.1.0"
+edition = "2024"
+description = "A cross-platform TUI playground for learning relational databases."
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/sturm/rdbms-playground"
+readme = "README.md"
+publish = false
+
+[dependencies]
+anyhow = "1.0.102"
+crossterm = { version = "0.29.0", features = ["event-stream"] }
+futures-util = "0.3.32"
+ratatui = "0.30.0"
+thiserror = "2.0.18"
+tokio = { version = "1.52.2", features = ["full"] }
+tracing = "0.1.44"
+tracing-subscriber = { version = "0.3.23", features = ["env-filter"] }
+
+[dev-dependencies]
+insta = { version = "1.47.2", features = ["yaml"] }
+pretty_assertions = "1.4.1"
+
+[lints.rust]
+unsafe_code = "forbid"
+unreachable_pub = "warn"
+
+[lints.clippy]
+all = { level = "warn", priority = -1 }
+nursery = { level = "warn", priority = -1 }
+# Allow common false-positives that don't materially improve our code.
+module_name_repetitions = "allow"
+missing_errors_doc = "allow"
+missing_panics_doc = "allow"