Persist the expert_mode preference the frontend already writes

[bdraco]

What does this implement/fix?

The frontend's Expert Mode toggle writes expert_mode via config/set_preferences, but the backend UserPreferences model never had the field, so mashumaro dropped it on write and get_preferences never returned it; the toggle reset to off on every reload. Adds expert_mode: bool to the model so the existing get/set path persists it. Backend only; the frontend already reads and writes the field.

Related issue or feature (if applicable):

• N/A

Types of changes

• Bugfix (non-breaking change which fixes an issue) — bugfix
• New feature (non-breaking change which adds functionality) — new-feature
• Enhancement to an existing feature — enhancement
• Breaking change (fix or feature that would cause existing functionality to not work as expected) — breaking-change
• Refactor (no behaviour change) — refactor
• Documentation only — docs
• Maintenance / chore — maintenance
• CI / workflow change — ci
• Dependencies bump — dependencies

Frontend coordination

• No frontend change needed
• Companion frontend PR: esphome/device-builder-frontend#

Checklist

• The code change is tested and works locally.
• Pre-commit hooks pass (ruff, codespell, yaml/json/python checks).
• Tests have been added or updated under tests/ where applicable.
• components.index.json / definitions/components/*.json have not been hand-edited (regenerate via script/sync_components.py if a sync is needed).
• Architecture-level changes are reflected in docs/ARCHITECTURE.md and/or docs/API.md.

[codspeed-hq]

Merging this PR will not alter performance

✅ 27 untouched benchmarks

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_{Comparing persist-expert-mode-preference (7f85dd9) with main (d6054e3)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.50%. Comparing base (d6054e3) to head (7f85dd9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1480   +/-   ##
=======================================
  Coverage   99.50%   99.50%           
=======================================
  Files         222      222           
  Lines       17253    17254    +1     
=======================================
+ Hits        17168    17169    +1     
  Misses         85       85           

Flag	Coverage Δ
py3.12	99.47% <100.00%> (+<0.01%)	⬆️
py3.14	99.50% <100.00%> (-0.01%)	⬇️

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

Files with missing lines	Coverage Δ
esphome_device_builder/models/preferences.py	100.00% <100.00%> (ø)

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[bdraco]

folded into #1445

final wire shape to be BEGINNER = “beginner” or EXPERT = “expert” and drop UI = “ui”, / YAML = “yaml”