fix(proxy): prevent panic when upstream client builder rejects headers

[OmerFarukOruc]

Summary

• remove panic path in UpstreamClient::new when default client creation fails
• retry client construction without upstream proxy on builder failure
• add final fallback to Client::new() so proxy service can continue instead of crashing
• validate default User-Agent header value before applying it to client builders, and fallback to a safe UA when invalid

Why

Issue #2004 reports runtime panic:

• thread 'tokio-runtime-worker' ... Failed to create default HTTP client: rquest::Error { kind: Builder, source: http::Error(InvalidHeaderValue) }

This change hardens startup to avoid panicking on InvalidHeaderValue and keeps proxy service alive with safe fallbacks.

Verification

• cargo test --manifest-path src-tauri/Cargo.toml test_build_url -- --nocapture
• result: build currently fails in this environment before reaching tests due boring-sys2 bindgen toolchain issue (fatal error: 'stddef.h' file not found), unrelated to this patch
• manually verified logic path: panic-causing expect(...) removed and replaced with non-panicking fallback handling

Closes #2004