packs: improve local addon import validation messages

[Senzo13]

Context

Local addons are important for extensibility, but pack import errors should be actionable for people creating their first pack.

Relevant files:

• core/infra/packs.py
• web/routes/settings.py
• web/static/js/settings.js
• tests/test_packs_smoke.py

Suggested approach

Improve errors for missing pack.json, invalid JSON, missing referenced asset files, unsupported kind, and duplicate pack IDs.

Acceptance criteria

• Backend errors identify the exact missing/invalid file when possible.
• UI displays a concise human-readable message.
• At least one test covers an invalid pack folder.
• Existing pack smoke tests pass.