fix(routes): migrate login page state to Svelte 5 runes

Replace plain let bindings with $state() runes in login form state;
ignore GitService runtime repos (backend/git_repos) in .gitignore.

.gitignore

@@ -113,3 +113,6 @@ axiom-mcp-tools-audit-report.md
 *.docx
 backend/relative
 .kilo/plans
+
+# GitService runtime repos (test artifacts, lock files)
+backend/git_repos/

frontend/src/routes/login/+page.svelte

@@ -23,10 +23,10 @@ import { SvelteURLSearchParams } from "svelte/reactivity";
     import { log } from '$lib/cot-logger';
     import { t } from '$lib/i18n/index.svelte.js';
 
-    let username = '';
-    let password = '';
-    let error = '';
-    let loading = false;
+    let username = $state('');
+    let password = $state('');
+    let error = $state('');
+    let loading = $state(false);
     let returnUrl: string | null = $state(null);
 
     // Read returnUrl from query params on mount