feat: rename Device and Instance parent route names to kebab-case

[cstns]

Closes #8189
Part of #8084

Summary

• Renames Device to device and Instance to instance in their route definitions
• Updates all 29 references across 21 files
• These two parent route names were missed from the sub-issue breakdown, all their child routes were already kebab-case

Test plan

• Grep for old names as quoted strings confirms no remaining references
• Navigation to device and instance pages works as before
• Once feat: add lint rule to enforce kebab-case route names #8107 is rebased on top of this, the route naming lint test should pass

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.16%. Comparing base (a24ab9a) to head (916140b).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8191      +/-   ##
==========================================
- Coverage   76.19%   76.16%   -0.04%     
==========================================
  Files         442      442              
  Lines       23678    23694      +16     
  Branches     6299     6306       +7     
==========================================
+ Hits        18042    18047       +5     
- Misses       5636     5647      +11     

Flag	Coverage Δ
backend	76.16% <ø> (-0.04%)	⬇️

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[n-lark]

I pushed a fix for a missed Instance -> instance in ChangeInstanceType - otherwise looks good!

• Grep for old names as quoted strings confirms no remaining references
• Navigation to device and instance pages works as before