Fix Sentry DSN startup crash; rewrite README for users

[Copilot]

APK crashes on startup with ArgumentException: Invalid DSN: No public key provided because SentrySdk.Init is called with placeholder DSN "https://localhost" on deployed builds where IsDeployedBuild is true (version major > 0).

Changes

• SentryLogger.cs: Wrap SentrySdk.Init in try-catch (ArgumentException) so invalid/placeholder DSNs fail gracefully instead of crashing the app

try
{
    sentrySession = SentrySdk.Init(options =>
    {
        options.Dsn = "https://localhost";
        // ...
    });
}
catch (ArgumentException ex)
{
    Logger.Log($"Failed to initialise Sentry SDK: {ex.Message}", LoggingTarget.Runtime, LogLevel.Debug);
    return;
}

• README.md: Full rewrite targeting end users — plain-language feature descriptions, prominent download section, technical details collapsed into <details> blocks. Covers all fork differences vs upstream (Oboe audio, S Pen/keyboard/mouse input, performance mode, 120Hz+, DeX, Vulkan detection, stability fixes, build toolchain, framework/Veldrid forks, CI/CD).

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[Copilot]

Pull request overview

Fixes an Android startup crash caused by Sentry initialisation with an invalid/placeholder DSN in deployed builds, and updates the repository README to be end-user focused with technical details folded into expandable sections.

Changes:

• Guard SentrySdk.Init() with an ArgumentException catch to avoid crashing when DSN is invalid.
• Rewrite README.md with clearer user-facing download/install guidance and concise feature explanations.
• Move deep technical details (toolchain, framework/Veldrid forks, CI) into <details> blocks.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
osu.Game/Utils/SentryLogger.cs	Prevents startup crash by handling invalid Sentry DSN initialisation gracefully.
README.md	Reorients documentation toward end users (download first), while preserving technical details in collapsible sections.

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