Settings: configurable storage location for request data

[syncended]

Goal

Introduce a settings layer for yapi. The first setting to support: where request data (saved requests, history, etc.) is stored on disk.

Scope

• Settings mechanism (file format + load/save) — pick a sensible default location (e.g. ~/.config/yapi/ on Unix).
• Setting: storage path for request-related data.
  • Default value when unset.
  • Override via config file.
  • Override via CLI flag / env var (TBD which).
• Apply the configured path consistently across CLI and TUI.

Out of scope (later issues)

• Other settings (theme, keybindings, proxies, etc.) — add as separate issues when needed.
• UI screen for editing settings inside the TUI.

Acceptance criteria

• User can change where request data is saved without editing source code.
• Both CLI and TUI honor the configured path.
• Default behavior works out of the box with no config file present.

[syncended]

Decisions (roadmap planning, 2026-04-27)

• Config format: TOML — `config.toml` at `<storage.path>/config.toml` (default `~/.config/yapi/` on Unix). Rationale: human-friendly for hand editing/review, established in CLI tooling (Cargo, gh, helix).
• Saved requests format: `.http` (one file = one request, nested directories as groups). Tracked separately in Saved requests on disk: .http format, 1 file = 1 request, dirs as groups #11.
• Override precedence: CLI flag > env var > config file > defaults. Concrete flag/env names TBD during implementation.

Default settings to expose first:

• `storage.path` (this issue's main deliverable).
• `http.timeout_ms`, `http.follow_redirects`, `http.verify_ssl`.
• `defaults.headers` (map merged into every request).