Implement bounty details page

[AgentGoose32]

Closes #4

/claim

Implements a fuller bounty details page with a dummy-but-complete UI.

Changes:

• Expands the details page header with status, difficulty, title, description, and reward
• Adds a code panel viewer section with repository link and formatted code sample
• Adds submission checklist guidance for hunters
• Adds owner/reward sidebar and submit bug report call-to-action
• Keeps API loading behavior and uses a fallback bounty so the page can be viewed as dummy UI if the backend request fails
• Replaces the previous minimal title/description-only details screen

Validation:

• git diff --check

Notes:

• npm --prefix frontend run build is currently blocked by existing project-wide TypeScript dependency/lint errors unrelated to this change, including missing react-icons/* type resolution and many pre-existing unused imports across other components.