Add web search autosave and usage preview data

[designerzhi]

Description

Summary

This PR contains Settings behavior changes and local-browser preview support.

Changes

• Web Search settings now auto-save:

  • provider changes save automatically
  • API keys save on blur or Enter
  • removed the section-level Save button

• Added lightweight Web Search API key validation:

  • required-key providers show “API key required” when empty
  • Brave keys must start with BSA
  • Tavily keys must start with tvly-
  • invalid formats show “Check the key format”

• Added inline Web Search save status:

  • Saving...
  • Saved
  • Failed to save
  • validation messages

• Simplified provider-specific UI:

  • providers that do not require an API key, such as Parallel, no longer show an API Key row

• Added browser-development Usage mock data:

  • realistic spend, budget, sessions, messages, tokens, tool calls
  • model breakdown sample data
  • helps preview Usage UI in local browser without requiring live gateway data

Validation

• Verified changed files have no editor-reported errors.
• Tested Web Search behavior in local browser preview:
  • Parallel hides API key row
  • Brave without key shows required state
  • invalid Brave prefix shows format warning
  • plausible key saves successfully
• Verified Usage page renders populated preview data locally.

Related Issue

Changes

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• CI/CD or build configuration change
• Refactoring (no functional changes)

Testing

• Tests pass locally (cd desktop && npm run test)
• Renderer tests pass locally (cd desktop/renderer && npm run test)
• Build succeeds (cd desktop && npm run build)
• Manual testing performed (describe below)

Checklist

• My code follows the coding guidelines of this project
• I have performed a self-review of my code
• I have commented my code where necessary
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or my feature works
• New and existing unit tests pass locally with my changes