agent: fix — auto-grant microphone permission on WebKitGTK/Linux

WebKitGTK denies getUserMedia by default with no permission prompt.
Connect to the permission-request signal and auto-allow, plus enable
media-stream and media-capabilities in WebKit settings.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -36,4 +36,7 @@ serde_json = "1"
 tokio = { version = "1", features = ["rt", "sync"] }
 arboard = "3.6.1"
 tauri-plugin-mcp = "0.7.1"
+
+[target.'cfg(target_os = "linux")'.dependencies]
+webkit2gtk = "2.0"
 sqlx = { version = "0.8", features = ["sqlite", "runtime-tokio"] }