chore: add toast messages for ai chat interactions

[richardshiue]

requires #7083

1. error toast when trying to open a page that no longer exists
2. success toast when finished saving a message to a page

Feature Preview

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PR Checklist

• My code adheres to AppFlowy's Conventions
• I've listed at least one issue that this PR fixes in the description above.
• I've added a test(s) to validate changes in this PR, or this PR only contains semantic changes.
• All existing tests are passing.

[github-actions]

🥷 Ninja i18n – 🛎️ Translations need to be updated

Project /project.inlang

lint rule	new reports	level	link
Missing translation	56	warning	contribute (via Fink 🐦)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 57.63%. Comparing base (38c2937) to head (bcf8ea0).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7086      +/-   ##
==========================================
- Coverage   57.67%   57.63%   -0.05%     
==========================================
  Files         320      320              
  Lines       14033    14043      +10     
  Branches     2040     2038       -2     
==========================================
  Hits         8094     8094              
- Misses       5675     5686      +11     
+ Partials      264      263       -1     

Flag	Coverage Δ
appflowy_web_app	57.63% <ø> (-0.05%)	⬇️

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