Add onboarding profile personalization

[FuJacob]

Summary

Adds a profile step to onboarding and Settings so Tabby can store the user's name plus common writing tags for personalized completions.

The profile fields now flow through the existing single autocomplete prompt path for both local llama and Apple Intelligence. The old local Completion Style split (Guided / Fast) was removed from this branch so the PR no longer reintroduces prompt-mode settings or mode-specific request behavior.

Validation

• swiftlint .
  • Found 0 violations, 0 serious in 79 files.
• git diff --check origin/main...HEAD
  • Exit 0.
• xcodebuild test -scheme tabby -destination 'platform=macOS' CODE_SIGNING_ALLOWED=NO
  • TEST SUCCEEDED; 148 tests, 0 failures.

Linked issues

None.

Risk / rollout notes

• This replaces the previous freeform custom-instructions setting with structured profile fields (userName, userTags). Existing users with old tabbyCustomAIInstructions data will not have that value mapped into the new profile fields.
• The branch was rebased onto current origin/main and force-pushed with a single cleaned-up commit.
• The unsigned test run still prints the existing llama/Metal shutdown assert after XCTest completes, but xcodebuild exits successfully and reports TEST SUCCEEDED.