fix: remove default Client ID per Spotify policy change

[0Chencc]

Summary

Spotify's new Development Mode rules (effective 2025-02-11) limit each Client ID to 5 authorized users. Shipping a shared public Client ID is no longer viable.

• Removed hardcoded spotifyClientId from AppConfig
• effectiveSpotifyClientIdProvider no longer falls back to any default — returns empty string if not configured
• Login screen: Client ID input is always visible and required before the Connect button is enabled
• Settings page: Client ID section promoted from hidden "Advanced Options" to a top-level editable field
• L10n: Updated descriptions in zh/en/ja to reflect this is a requirement, not optional

Test plan

• Fresh install: login button disabled until Client ID is saved
• Enter Client ID → save → login button enabled → auth works
• Settings: can view and change Client ID (prompts re-login)
• App restart: saved Client ID persists and login works

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.42%. Comparing base (0a71bfc) to head (7bf07d3).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #14   +/-   ##
=======================================
  Coverage   71.42%   71.42%           
=======================================
  Files           1        1           
  Lines           7        7           
=======================================
  Hits            5        5           
  Misses          2        2           

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