fix(A.1 #2): parse tauri.conf.json properly in CSP regression guard

Review feedback: the original guard substring-searched the whole file
after the first "csp" token, which (a) false-passes if any unrelated
JSON value elsewhere in the config happens to contain a localhost URL
and (b) false-fails if the CSP is ever re-serialised with escaped
forward slashes.

Switches to serde_json + a /app/security/csp pointer lookup, then
splits the CSP on ';', finds the connect-src directive, tokenises its
allow-list on whitespace, and requires an exact match for both
http://127.0.0.1:* and ws://127.0.0.1:*. The error now also includes
the current connect-src value so a developer who breaks it can see
exactly what needs restoring.

src-tauri/build.rs

@@ -21,23 +21,46 @@ fn main() {
 /// `fetch()` from the webview to the local LLM silently 404s with an
 /// opaque scope error (Vibe #438 / #487). We keep the current permit
 /// pinned at build time so a stray edit can't regress it unnoticed.
+///
+/// Parses `tauri.conf.json` properly and inspects the `connect-src`
+/// directive of the CSP, rather than substring-searching the whole
+/// file — the latter would both false-pass on unrelated values
+/// containing a localhost URL and false-fail on JSON-escaped forward
+/// slashes.
 fn assert_localhost_llm_csp() {
     let conf_path = std::path::Path::new("tauri.conf.json");
     println!("cargo:rerun-if-changed=tauri.conf.json");
 
     let raw = std::fs::read_to_string(conf_path)
         .expect("build.rs: failed to read tauri.conf.json for CSP regression guard");
-    let csp_start = raw
-        .find("\"csp\"")
-        .expect("build.rs: tauri.conf.json missing \"csp\" field");
-    // Read the rest of the CSP string — we only need to substring-search.
-    let csp_slice = &raw[csp_start..];
+    let conf: serde_json::Value = serde_json::from_str(&raw)
+        .expect("build.rs: tauri.conf.json is not valid JSON");
+
+    let csp = conf
+        .pointer("/app/security/csp")
+        .and_then(|v| v.as_str())
+        .expect("build.rs: tauri.conf.json missing app.security.csp string");
+
+    // Split the CSP into directives (`default-src 'self'; script-src ...`)
+    // and find connect-src. Everything after the directive name up to the
+    // next `;` is the allow-list.
+    let connect_src = csp
+        .split(';')
+        .map(str::trim)
+        .find_map(|directive| directive.strip_prefix("connect-src"))
+        .map(str::trim)
+        .expect(
+            "build.rs: tauri.conf.json CSP missing connect-src directive — \
+             local LLM fetches will be blocked (brief item #2)",
+        );
+
+    let tokens: Vec<&str> = connect_src.split_whitespace().collect();
     for required in ["http://127.0.0.1:*", "ws://127.0.0.1:*"] {
         assert!(
-            csp_slice.contains(required),
-            "build.rs: tauri.conf.json CSP must permit {required} for local LLM \
-             connectivity (brief item #2). Restore the connect-src entry before \
-             building."
+            tokens.iter().any(|t| *t == required),
+            "build.rs: tauri.conf.json CSP connect-src must permit {required} \
+             for local LLM connectivity (brief item #2). Current connect-src: \
+             {connect_src:?}"
         );
     }
 }