Add find-in-page (Cmd+F) for browser panels (#837) (#875)

JavaScript-based find using TreeWalker + <mark> highlights with
match counter, next/previous navigation, and drag-to-corner overlay
matching the existing terminal find bar.

- BrowserFindJavaScript: JS generation for search/next/prev/clear
- BrowserSearchOverlay: SwiftUI overlay with IME-safe onSubmit
- BrowserSearchState: Observable state (needle/selected/total)
- TabManager routing: Cmd+F/G dispatches to browser when focused
- Visibility filter: skips script/style/hidden/aria-hidden elements
- Stale DOM guard: isConnected check in next/previous scripts
- Navigation cleanup: clears find on didFinish and didFailNavigation

Co-authored-by: Lawrence Chen <54008264+lawrencecchen@users.noreply.github.com>

cmuxTests/BrowserFindJavaScriptTests.swift

@@ -0,0 +1,116 @@
+import XCTest
+
+#if canImport(cmux_DEV)
+@testable import cmux_DEV
+#elseif canImport(cmux)
+@testable import cmux
+#endif
+
+final class BrowserFindJavaScriptTests: XCTestCase {
+
+    // MARK: - searchScript
+
+    func testSearchScriptReturnsNonEmptyJavaScript() {
+        let js = BrowserFindJavaScript.searchScript(query: "hello")
+        XCTAssertFalse(js.isEmpty)
+        XCTAssertTrue(js.contains("hello"))
+    }
+
+    func testSearchScriptEmptyQueryReturnsEarlyReturn() {
+        let js = BrowserFindJavaScript.searchScript(query: "")
+        XCTAssertTrue(js.contains("total: 0"))
+    }
+
+    // MARK: - nextScript / previousScript
+
+    func testNextScriptReturnsValidJavaScript() {
+        let js = BrowserFindJavaScript.nextScript()
+        XCTAssertFalse(js.isEmpty)
+        XCTAssertTrue(js.contains("__cmuxFindMatches"))
+    }
+
+    func testPreviousScriptReturnsValidJavaScript() {
+        let js = BrowserFindJavaScript.previousScript()
+        XCTAssertFalse(js.isEmpty)
+        XCTAssertTrue(js.contains("__cmuxFindMatches"))
+    }
+
+    // MARK: - clearScript
+
+    func testClearScriptReturnsValidJavaScript() {
+        let js = BrowserFindJavaScript.clearScript()
+        XCTAssertFalse(js.isEmpty)
+        XCTAssertTrue(js.contains("__cmux-find"))
+    }
+
+    // MARK: - jsStringEscape
+
+    func testEscapesDoubleQuotes() {
+        let result = BrowserFindJavaScript.jsStringEscape(#"say "hello""#)
+        XCTAssertEqual(result, #"say \"hello\""#)
+    }
+
+    func testEscapesBackslashes() {
+        let result = BrowserFindJavaScript.jsStringEscape(#"path\to\file"#)
+        XCTAssertEqual(result, #"path\\to\\file"#)
+    }
+
+    func testEscapesNewlines() {
+        let result = BrowserFindJavaScript.jsStringEscape("line1\nline2")
+        XCTAssertEqual(result, "line1\\nline2")
+    }
+
+    func testEscapesCarriageReturns() {
+        let result = BrowserFindJavaScript.jsStringEscape("line1\rline2")
+        XCTAssertEqual(result, "line1\\rline2")
+    }
+
+    func testEscapesTabs() {
+        let result = BrowserFindJavaScript.jsStringEscape("col1\tcol2")
+        XCTAssertEqual(result, "col1\\tcol2")
+    }
+
+    func testPlainTextPassesThrough() {
+        let result = BrowserFindJavaScript.jsStringEscape("hello world 123")
+        XCTAssertEqual(result, "hello world 123")
+    }
+
+    func testJapaneseTextPassesThrough() {
+        let result = BrowserFindJavaScript.jsStringEscape("こんにちは")
+        XCTAssertEqual(result, "こんにちは")
+    }
+
+    func testMixedSpecialCharacters() {
+        let result = BrowserFindJavaScript.jsStringEscape(#"a\"b\nc"#)
+        XCTAssertEqual(result, #"a\\\"b\\nc"#)
+    }
+
+    func testEscapesNullByte() {
+        let result = BrowserFindJavaScript.jsStringEscape("a\0b")
+        XCTAssertEqual(result, "a\\0b")
+    }
+
+    func testEscapesLineSeparator() {
+        let result = BrowserFindJavaScript.jsStringEscape("a\u{2028}b")
+        XCTAssertEqual(result, "a\\u2028b")
+    }
+
+    func testEscapesParagraphSeparator() {
+        let result = BrowserFindJavaScript.jsStringEscape("a\u{2029}b")
+        XCTAssertEqual(result, "a\\u2029b")
+    }
+
+    // MARK: - searchScript escaping integration
+
+    func testSearchScriptEscapesQueryInOutput() {
+        let js = BrowserFindJavaScript.searchScript(query: #"test"injection"#)
+        // The double quote should be escaped, not breaking the JS string literal.
+        XCTAssertTrue(js.contains(#"test\"injection"#))
+        XCTAssertFalse(js.contains(#"test"injection"#))
+    }
+
+    func testSearchScriptHandlesLineSeparator() {
+        let js = BrowserFindJavaScript.searchScript(query: "test\u{2028}break")
+        XCTAssertTrue(js.contains("\\u2028"))
+    }
+}