Add crypto_market.py: refactor duplicated CoinGecko API patterns into shared helpers

[Copilot]

The repository had no source code. This PR introduces crypto_market.py, a CoinGecko API utility module structured from the ground up to avoid duplication.

Refactoring highlights

• _get(endpoint, params) — single HTTP call + error handling helper; replaces the repeated requests.get / raise_for_status / .json() triad that would otherwise appear in every function
• _format_price(coin_id, price, currency) — single formatting definition instead of inline f-strings scattered across display functions
• Parameterised fetch — get_coin_price(coin_id) and get_prices(coin_ids) replace per-coin duplicate functions with identical bodies:

# Instead of:
def get_bitcoin_price():
    url = "https://api.coingecko.com/api/v3/simple/price?ids=bitcoin&vs_currencies=usd"
    response = requests.get(url); response.raise_for_status()
    return response.json()["bitcoin"]["usd"]

def get_ethereum_price():   # identical structure, different strings
    ...

# Refactored to:
def get_coin_price(coin_id, vs_currency="usd"):
    data = _get("/simple/price", params={"ids": coin_id, "vs_currencies": vs_currency})
    ...
    return data[coin_id][vs_currency]

• Loop-based display — display_prices / display_market_summary iterate over a coin list rather than per-coin copy-paste blocks

Error handling

_get emits a descriptive error including the endpoint URL and HTTP status code; get_coin_price and get_prices raise ValueError with the specific coin/currency missing from the response rather than an opaque KeyError.

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