Tauri migration (M1): shell + web UI scaffold side-by-side with egui

[tharinfernando]

Goal

Easy-to-update, cross-platform UI (Linux/macOS/Windows) per plan. This is Milestone 1: prove the Tauri IPC + shared-core + theming path while the egui app keeps working untouched.

What lands here

src-tauri/ (new termdb-tauri workspace member)

• ping — webview↔Rust round-trip proof
• list_connections — reads the same SQLite ConfigStore as egui (zero core changes)
• get_palette — Omarchy colors.toml reader ported from theme.rs, slate fallback off-Linux
• tauri.conf.json (1200×800, all bundle targets), least-privilege capabilities/default.json, placeholder RGBA icons (tauri icon follow-up before release)

frontend/ (Svelte 5 + TS + Vite)

• Typed invoke wrappers mirroring termdb-core models (hand-written until tauri-specta lands)
• TopBar / Sidebar components, CSS-variable theming from get_palette()
• Table / Query Editor / Connection form are marked placeholders for M2–M3

Verification

• npm run build — svelte-check 0 errors, vite build ok
• cargo check -p termdb-tauri, cargo test -p termdb-tauri (3 passed), fmt --check, clippy clean
• cargo check --workspace — egui + core + Tauri compile together

Run it

npm --prefix frontend run build  # once: generate_context! reads frontend/dist
cargo run -p termdb-tauri
cargo run -p termdb              # old egui app, unchanged

Follow-ups (not in this PR)

• M2: engine extraction → connect/list_tables/open_table/load_page + sidebar tree + DataGrid + connection form
• M3: record dialog, CodeMirror SQL editor, history/results, CSV/JSON export
• Real icons via tauri icon, tauri-specta bindings, CI job (frontend build → cargo test → tauri build)

Kept open as work-in-progress migration branch — do not merge yet.