fix: add backend onboarding and safe switching

[christopherklint97]

Problem

The Apps2Samsung widget for StreamVault was built with a private build-machine backend URL (http://10.1.0.200:3002). Users could enter Xtream credentials, but the widget could not reach a StreamVault backend and only reported Failed to fetch.

Changes

• stop deriving public widget backend URLs from the builder's LAN address
• retain explicit VITE_SERVER_URL / VITE_SERVER_IP support for controlled builds
• add backend-first Settings onboarding for local Tizen widgets
• support optional STREAMVAULT_AUTH_TOKEN during remote and same-origin onboarding
• probe candidate backends without forwarding the active backend's token
• commit backend switches transactionally only after status, config, and program bootstrap succeeds
• preserve the active backend when a candidate connection fails
• replace opaque browser network failures with an actionable StreamVault backend error
• guard backend-scoped requests, polling, recording mutations, and ABA races by generation
• stop active HTML5, mpegts.js, and Tizen AVPlay playback when changing backends
• reset backend-scoped views, recordings, favorites, recent items, and watch progress when backend identity changes
• prevent the PWA service worker from caching bootstrap and authenticated recording endpoints
• fix early Tizen logging for JSON-encoded runtime backend URLs
• document the separate StreamVault backend requirement and token workflow

The broken Apps2Samsung package was separately retired in Apps2Samsung/tizen-community-packages#43.

Verification

• npm audit --omit=dev
• npm test — 40 files, 212 tests
• npm run lint
• npm run typecheck
• npm run build
• npm run build:tizen5
• generated artifact scan — no embedded private backend URL
• cd server && npm test — 29 files, 172 tests
• cd server && npm run typecheck
• cd server && npm run audit:prod
• docker build -t streamvault:issue37-final .
• independent final review — no findings

Risk

A successful switch to a different backend URL clears client-side favorites, recent items, and watch progress because channel identifiers are backend-specific. Reconnecting to the same backend preserves them.

Closes #37