feat(networking): add flavor to routers

[hgw77]

Summary

Integrates Neutron Network Flavors into the router creation and management workflow in Elektra, allowing users to discover
and select flavors directly from the UI instead of requiring CLI access.

Changes Made

• Service layer: Added NetworkingServices::Flavor module with flavors() and find_flavor() methods (graceful fallback on API error); included in NetworkingService
• Model: Added Networking::Flavor model; added flavor_name transient attribute and VPNaaS AZ validation to Networking::Router
• Router creation form: Added optional Network Flavor dropdown (filtered to L3_ROUTER_NAT, enabled only); immutable-flavor warning shown when a flavor is selected; Availability Zone field with dynamic required marker and VPNaaS hint; Internal Subnets field hidden when VPNaaS flavor is selected
• Router edit form: Flavor field and info banner shown when router has a VPNaaS flavor; Internal Subnets hidden with explanation
• Router detail view: Network Flavor shown in Overview tab (name, description, UUID); falls back to "None"
• Router list view: Flavor badge with description tooltip; status rendered as colored badge (ACTIVE=green, ERROR=red, DOWN=grey)
• JavaScript: handleFlavorChange() in router.js handles dynamic VPNaaS UI logic (AZ required marker, hint, internal subnets visibility, immutable warning)

Related Issues

• Issue: [Feature](network): flavors for routers #2130

Screenshots

Checklist

• I have performed a self-review of my code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have made corresponding changes to the documentation (if applicable).
• My changes generate no new warnings or errors.