Improve bounty detail page clarity

[Zejia]

Closes #4

This makes the bounty detail page easier to scan before someone starts work:

• separates status, reward, and source issue into summary cards
• keeps the GitHub issue link prominent
• moves acceptance text into its own short section
• adds a regression test for the key fields on the detail page

Checks run:

• ./.venv/bin/python -m pytest
• ./.venv/bin/python -m ruff format --check .
• ./.venv/bin/python -m ruff check .
• ./.venv/bin/python -m mypy app