feat: configurable API endpoints via {PROVIDER}_BASE_URL convention

[Star-Lotus]

Summary

• Add unified endpoint registry (src/utils/endpoints.py) with get_provider_base_url() utility and PROVIDER_DEFAULTS dict
• Replace all hardcoded API URLs in 6 model adapters (DashScope, Kling, Vidu) with env-var-driven lookups
• Add "Advanced: API Endpoints" collapsible section in EnvConfigDialog for UI-based endpoint configuration
• Convention: {PROVIDER}_BASE_URL env var overrides default, empty string falls back to default

Motivated by PR #4's idea of endpoint configurability, but implemented with a convention-over-configuration approach. New provider onboarding = 3 lines of code (one in PROVIDER_DEFAULTS, one in model class, one in frontend registry).

Changes

Backend:

• New src/utils/endpoints.py — provider registry + get_provider_base_url() with trailing-slash stripping and empty-string fallback
• wanx.py, image.py, qwen_vl.py, llm_adapter.py, kling.py, vidu.py — replaced hardcoded URLs
• api.py — EnvConfig.endpoint_overrides dict with allowlist validation, remove_user_config_keys() for proper override clearing, deduplicated GET route

Frontend:

• EnvConfigDialog.tsx — ENDPOINT_PROVIDERS registry, collapsible endpoint section, safe ?? {} fallback
• api.ts — updated saveEnvConfig type signature

Tests & Config:

• 28 new vitest tests covering validation, state transforms, data normalization, and registry structure
• .env.example documents {PROVIDER}_BASE_URL convention with international endpoint examples

Test plan

• No env vars set → all providers use default URLs (backward compatible)
• DASHSCOPE_BASE_URL=https://test.example.com → requests go to new endpoint
• Empty string override → falls back to default
• Trailing slash in URL → stripped automatically
• EnvConfigDialog loads/saves endpoint overrides correctly
• Unknown keys in endpoint_overrides rejected (allowlist validation)
• Override removal persisted to config file (.env / JSON)
• 56 frontend tests passing (28 endpoint + 28 video-params)
• All Python module imports verified
• TypeScript compilation: zero errors in changed files

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