Document iOS 13 deployment target

[leehack]

Summary

• Update active README/docs guidance from iOS 16.4 to iOS 13.0.
• Lower the checked-in chat app Podfile and Runner deployment target settings to 13.0.
• Add an Unreleased changelog note for the deployment-target change.

Validation

• git diff --check
• npm ci in website/
• ./tool/docs/validate_links.sh

Notes

This is paired with leehack/llamadart-native#16, which rebuilds the llama.cpp native runtime for the lower iOS deployment target. Keep this draft until the native runtime PR/release path is ready.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.57%. Comparing base (a0f6b4c) to head (b5a8abb).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #197   +/-   ##
=======================================
  Coverage   80.57%   80.57%           
=======================================
  Files          85       85           
  Lines       11392    11392           
=======================================
  Hits         9179     9179           
  Misses       2213     2213           

Flag	Coverage Δ
unittests	80.57% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[leehack]

Closing this draft PR for now; the current iOS 13 documentation/update path is being discarded.

[github-actions]

Chat app preview removed for leehack/llamadart-chat-pr-197.