feat: add configurable share URL for self-hosted portals

[backnotprop]

Summary

• Adds PLANNOTATOR_SHARE_URL env var so users can self-host the share portal and have all generated share links point to their own domain
• Threads the custom base URL through the same path as sharingEnabled: env var → server options → API response → editor state → useSharing hook → generateShareUrl()
• Adds apps/portal/SELF-HOSTING.md with build, deploy, and configuration instructions
• Import dialog placeholder updates dynamically to reflect the configured domain

Closes #12

Test plan

• No env var set → share URLs default to https://share.plannotator.ai
• PLANNOTATOR_SHARE_URL=https://custom.domain → share URLs start with configured domain
• Import modal placeholder reflects configured domain
• bun run build:hook && bun run build:opencode && bun run build:portal all pass
• Portal self-hosts correctly: build, serve dist/, open a generated share link

🤖 Generated with Claude Code

[backnotprop]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

🤖 Generated with Claude Code

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