feat: consolidate web_ui_ config fields under web_ui section

[akorobkov-nvda]

Is this a new feature, an enhancement, or a change to existing functionality?

Enhancement

How would you describe the priority of this feature request

Low (would be nice)

Please provide a clear description of problem this feature solves

The admin web UI configuration uses a flat naming convention with web_ui_ prefix:

web_ui_sidebar_tools = [...]
web_ui_logs_link_template = "..."

As more web UI settings are added, this flat structure becomes harder to maintain and document. A nested [web_ui] section would be cleaner and more consistent with other config sections

Feature Description

As an operator, I want related web UI configuration grouped under a [web_ui] section so that the config file is easier to navigate and understand.

Describe your ideal solution

Migrate from flat to nested structure:

# Before (current):
web_ui_sidebar_tools = [...]
web_ui_logs_link_template = "..."

# After (proposed):
[web_ui]
logs_link_template = "..."

[[web_ui.sidebar_tools]]
name = "grafana"
display_name = "Grafana"
url = "https://grafana.example.com"

1. Add new nested WebUiConfig struct with sidebar_tools and logs_link_template fields
2. Use #[serde(alias = "web_ui_sidebar_tools")] and #[serde(alias = "web_ui_logs_link_template")] to accept old field names during transition
3. Log deprecation warning when old flat fields are used
4. Update documentation to show new nested structure

Describe any alternatives you have considered

• Keep flat structure indefinitely - works but gets messier as more settings are added
• Hard break without aliases - too disruptive for existing deployments

Additional context

Affected files:

• crates/api-core/src/cfg/file.rs - Config struct definitions
• crates/api-core/src/cfg/load.rs - Config loading and validation
• crates/api-core/src/test_support/default_config.rs - Test defaults
• deploy/files/nico-api/nico-api-site-config.toml - Example config
• crates/api-core/src/cfg/README.md - Documentation

Related PRs:

• feat(web): make admin ui logs link configurable for different logging backends #4969 (feat: make admin ui logs link configurable for different logging backends #4954)
• feat(web): make admin UI Tools sidebar configurable via site config #1281

Code of Conduct

• I agree to follow NVIDIA Infra Controller's Code of Conduct
• I have searched the open feature requests and have found no duplicates for this feature request