Fix web view authentication challenges

[timbms]

Rebased on DigiH:settings.

Summary

• Adds alwaysSendSameAuthenticationToWebView to HomePreferences — when enabled, server credentials are forwarded to all web view authentication challenges regardless of host
• Automatically forwards credentials when the challenge host matches a configured server URL or is home.myopenhab.org, with no setting change required
• Adds a toggle in Connection Settings (visible when not in demo mode) to control the opt-in behaviour
• Adds a custom Codable decoder to HomePreferences using decodeIfPresent for all fields, ensuring existing users don't lose their preferences when new keys are introduced

Fixes #1034

Test plan

• Webview pointing to a resource on the same host as the openHAB server loads correctly without enabling the toggle
• Webview pointing to a resource on a different host (e.g. embedded Grafana on same IP but different path) loads correctly after enabling "Always send credentials to web views"
• Remote connection through home.myopenhab.org authenticates web views correctly without enabling the toggle
• Toggling the setting off reverts to default-handling for unmatched hosts
• Existing user preferences survive an app update (no data loss)