Fix: handle non-JSON Invidious API responses gracefully

[Houmgaor]

Summary

• When an Invidious instance returns non-JSON responses (e.g. plain text "API disabled"), the app currently crashes with SyntaxError: Unexpected token 'A', "API disabled" is not valid JSON
• This change reads the response as text first, checks response.ok, and wraps JSON.parse in a try/catch, so the actual server message is surfaced as the error instead of a cryptic JSON parse failure
• This is the single most commonly reported error in the app, affecting users whose Invidious instances have disabled their APIs

relates to #7883, #8527, #5720, #5506

Test plan

• Configure an Invidious instance that has its API disabled and verify the error message now shows the actual server response (e.g. "API disabled") instead of a JSON SyntaxError
• Verify normal Invidious API usage still works correctly with a working instance
• Verify that Invidious JSON error responses (e.g. {"error": "..."}) are still handled as before

[absidue]

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