Implement interactive onboarding wizard

[lstpsche]

Summary

Adds a new onboard subcommand (alias: init) that replaces the current manual config setup with a styled, interactive setup wizard.

Before this change, users had to manually create the config directory and write a TOML file by hand. Now they can run:

weathr onboard
# or
weathr init

What changed

New files:

• src/onboard.rs -- the full onboarding flow: prompts, geocoding integration, styled output, config save.

Modified files:

• Cargo.toml -- added dialoguer (interactive prompts with fuzzy-select).
• src/main.rs -- converted flat CLI to optional subcommand via clap's #[command(subcommand)]. Existing flags and behavior are untouched when no subcommand is given. Rustls crypto provider is now installed at startup to cover all HTTP code paths (geolocation, onboard, weather).
• src/config.rs -- added Serialize to Config and Location. Added save() (creates parent dirs automatically), get_config_dir() methods. Made get_config_path() and validate() public.
• src/weather/types.rs -- added Serialize to WeatherUnits.
• src/error.rs -- added OnboardError enum. Added WriteError and SerializeError variants to ConfigError.
• src/lib.rs -- registered the new onboard module.

New dependencies

Crate	Version	Purpose
dialoguer	0.12 (with fuzzy-select)	Select, FuzzySelect, Input, Confirm interactive prompts

Terminal styling uses crossterm::style::Stylize (already a project dependency). URL construction uses reqwest::Url::parse_with_params (re-exported from the url crate that reqwest already depends on). No additional direct dependencies needed for either.

How it works

The wizard runs in three phases:

Phase 1 -- Directory and file setup

Resolves the platform-specific config path and loads the existing config file if present. If the file exists but is corrupted, a warning is printed and defaults are used. Falls back to Config::default() if no file exists. Directory creation is handled by Config::save() automatically.

Phase 2 -- Interactive prompts

Walks through every config setting with styled prompts. Existing values are shown as defaults -- pressing Enter keeps them.

Prompt sequence:

1. Location method (Select): Enter coordinates / Search by city name / Use auto-detection
2. Coordinates (Input<f64> with validation) or City search (text input → Open-Meteo Geocoding API → FuzzySelect from results). City search automatically sets the city name and switches display mode to "City".
3. Auto-location (Confirm, coordinates path only): Enable IP-based auto-detection? Skipped after city search since the user explicitly chose a location.
4. Location display (Select): Coordinates only / City name / Both (city + coordinates)
5. City name language (Input): Language code for city names in the HUD (default: "auto" = system locale)
6. Hide location (Confirm): Hide coordinates in the UI?
7. Temperature unit (Select): Celsius / Fahrenheit
8. Wind speed unit (Select): km/h / m/s / mph / knots
9. Precipitation unit (Select): mm / inch
10. Hide HUD (Confirm): Hide the status line?
11. Silent mode (Confirm): Suppress non-error output?

A single reqwest::Client is created once and reused across city search calls within the wizard.

Phase 3 -- Save and finish

Validates the final config, serializes with toml::to_string_pretty(), writes to the config path (creating parent directories if needed), and prints a success message.

City search via Geocoding API

When the user chooses "Search by city name", the wizard:

1. Prompts for a city name (minimum 2 characters).
2. Calls the Open-Meteo Geocoding API (GET https://geocoding-api.open-meteo.com/v1/search?name=...&count=10&language=en).
3. Displays results as a fuzzy-searchable list formatted as City, Region, Country (lat, lon) - pop. N.
4. Includes a "-- Search again --" option at the bottom to retry with a different query.
5. Non-ASCII city names (e.g. "Москва", "München") are properly percent-encoded via reqwest::Url::parse_with_params.
6. On selection, populates city, sets display = "city", and preserves the existing city_name_language setting.

Sample output

Welcome banner and location prompt:

┌───────────────────────────────────────┐
│      Welcome to weathr setup!         │
│  Let's configure your weather app.    │
└───────────────────────────────────────┘

  Tip: existing values are shown as defaults. Press Enter to keep them.

── Location ────────────────────────────

? How would you like to set your location?
> Enter coordinates (latitude/longitude)
  Search by city name
  Use auto-detection (IP-based)

City search results:

? City name: Berlin
  Searching for "Berlin"...
? Select a city
> Berlin, Berlin, Germany (52.5244, 13.4105) - pop. 3426354
  Berlin, New Hampshire, United States (44.4688, -71.1854) - pop. 9293
  Berlin, Wisconsin, United States (43.9681, -88.9435) - pop. 5420
  -- Search again --

Location display and units:

? How should the location be displayed in the HUD? [current: city]
> Coordinates only
  City name
  Both (city + coordinates)
? City name language code ("auto" = system locale) [current: auto]: ↵
? Hide location coordinates in the UI? [current: no] (y/N)

── Units ───────────────────────────────

? Temperature unit [current: celsius]
> Celsius (°C)
  Fahrenheit (°F)

Completion:

── All set! ────────────────────────────

  Config saved to /home/user/.config/weathr/config.toml

  Run weathr to start!

Edge cases covered

Scenario	Behavior
Config directory doesn't exist	Created automatically by Config::save()
Config file doesn't exist	Uses Config::default(), creates file on save
Config file exists but is corrupted	Warns via stderr, falls back to Config::default()
Existing valid config	All current values shown as defaults; Enter keeps them
Invalid lat/lon input (e.g. 999)	Rejected by validator, re-prompts
City search returns no results	Prints error, loops back to city name prompt
City search -- user wants different query	"-- Search again --" option in results list
Network failure during city search	Prints error, breaks out of search loop, keeps current coordinates
Non-ASCII city names (Cyrillic, CJK, accented)	Proper percent-encoding via reqwest::Url
User presses Escape to cancel (any prompt)	Prints "Setup cancelled.", exits with code 0 (not an error)
Hard error during onboarding	Prints error message, exits with code 1

Backward compatibility

• All existing CLI flags and behavior are unchanged when no subcommand is given.
• Config loading precedence (local ./config.toml > XDG path) is unchanged.
• All existing tests pass. No test modifications needed.

Screenshots

Initial

City search

Example of units selection

Full onboarding log after completion