[Mastersam07]

Closes #91

The first screen inside a shell or module mount had no AppBar back arrow, though system back already left the shell. It now shows one when there's a page below the shell, and tapping it pops the shell. This makes a single branch shell a clean way to share a provider across a group of screens.

• Shells and KaiselModuleMount give their first page a local history entry, so AppBar shows the arrow and tapping it pops the hosting page.
• rootBackArrow: false opts out per shell or module mount.
• System back and Android predictive back are unchanged; new tests in kaisel_shell_root_back_test.dart.
• Shells guide: new section on sharing a provider across a group of screens.

[github-actions]

📖 Docs preview: https://kaisel.dev/pr-102/ (updates on every push; removed when this PR closes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.67%. Comparing base (c9dc4bf) to head (c1a561f).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #102      +/-   ##
==========================================
+ Coverage   99.66%   99.67%   +0.01%     
==========================================
  Files          43       43              
  Lines        2418     2496      +78     
==========================================
+ Hits         2410     2488      +78     
  Misses          8        8              

Flag	Coverage Δ
kaisel	99.67% <100.00%> (+0.01%)	⬆️
kaisel_core	99.67% <100.00%> (+0.01%)	⬆️
kaisel_devtools	99.67% <100.00%> (+0.01%)	⬆️
kaisel_lint	99.67% <100.00%> (+0.01%)	⬆️

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.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.