Improve 404/403 error page with clear messaging and next steps

[cqnykamp]

Summary

Fixes #2897 — the error page was showing raw technical messages (e.g. "Request failed with status code 404") as the heading, with no actionable next steps beyond "Back to Home".

• 404 (Content Not Found): Friendly heading + explanation that content may be private, with buttons to go to Homepage or Explore Public Activities
• 403 – private content: "This Content is Private" heading with guidance to ask the author to make it public, plus Homepage/Explore buttons
• 403 – not logged in: "Sign In Required" heading with a Sign In button
• Network/other errors: Generic "Page Unavailable" or "Something Went Wrong" with Homepage button
• Removes the unused SVG path arrays (unused eye/mouth variants) and extracts the sad face into its own SadFace component; shrinks it slightly so it doesn't dominate the page

Test plan

• Navigate to a non-existent activity URL and confirm "Content Not Found" heading appears with Homepage and Explore buttons
• Navigate to a private activity URL while logged out and confirm "Sign In Required" or "This Content is Private" (depending on API response) appears
• Verify the "Explore Public Activities" button navigates to /explore
• Verify the "Go to Homepage" button navigates to /
• Verify the data-test="Error Message" attribute is still present for Cypress tests

https://claude.ai/code/session_01DzrdWv57J7pTJFaYjg5CbD

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