Refactor: split lib.rs and main.js into modules; add unit tests for pure helpers

[astetic-dev]

Problem

src-tauri/src/lib.rs (~1700 lines) and src/main.js (~2000 lines) each hold five clearly separated domains (PTY/sessions, config/branding, dropper, TTS/STT, preview) in one file, and there are zero tests — while the pure helpers (mdToHtml/mdSafeUrl, build_command, unique_path, the hand-rolled b64, clampMode) are ideal unit-test targets.

Proposed fix

1. Rust: split into modules (config, pty, branding, speech, dropper) — mechanical, the section markers already exist.
2. JS: split main.js into ES modules (index.html already loads it with type="module").
3. Add unit tests for the pure helpers on both sides (Rust: cargo test; JS: needs the module split first).

Step 3 for Rust can land first; the rest is a larger mechanical refactor.

[astetic-dev]

Partially landed in 0.4.0: unit tests for the pure Rust helpers (11 tests, see #84). The module split itself (Rust modules + JS ES-module split with JS tests) deliberately did not make 0.4.0 and stays on the backlog.