fix(cr-2026-04-22): CSP guard matches connect-src as exact directive name

MINOR from the 2026-04-22 review (build.rs:47-64): the guard used
strip_prefix("connect-src") to find the directive, which would also
match hypothetical future directives like connect-src-elem and
validate the wrong allow-list.

Switches to split_once(char::is_whitespace) on each directive and
requires the first token to equal "connect-src" exactly. Robust
against prefix collisions with any future CSP3 directive
additions.

src-tauri/build.rs

@@ -42,13 +42,20 @@ fn assert_localhost_llm_csp() {
         .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.
+    // and find the one whose directive NAME is exactly "connect-src".
+    // A prefix match would also accept hypothetical future directives
+    // like "connect-src-elem" (2026-04-22 review MINOR).
     let connect_src = csp
         .split(';')
         .map(str::trim)
-        .find_map(|directive| directive.strip_prefix("connect-src"))
-        .map(str::trim)
+        .find_map(|directive| {
+            let (name, rest) = directive.split_once(char::is_whitespace)?;
+            if name == "connect-src" {
+                Some(rest.trim())
+            } else {
+                None
+            }
+        })
         .expect(
             "build.rs: tauri.conf.json CSP missing connect-src directive — \
              local LLM fetches will be blocked (brief item #2)",