feat(frontend): pathname-addressable views, deep links survive login, config page layout

Every view (/activities, /analysis, /plan, /profile, /config) is now
driven by the URL -- one pushState/popstate navigate() replaces the
tab/profile/config state flags, so all views deep-link, survive reload,
and honor Back/Forward. A logged-out visit to any path is stashed by
LoginGate and restored (replaceState) on the first render after the
OIDC round-trip, which otherwise always lands on '/'. The config page
gets Profile-style card spacing and a wider card so environment values
fit on one line.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/App.css

@@ -608,12 +608,19 @@ button:disabled {
   max-width: 580px;
 }
 
-.profile-page {
+.profile-page,
+.config-page {
   display: flex;
   flex-direction: column;
   gap: 1.75rem;
 }
 
+/* Wider than the default .kind-editor cap: the environment card lists
+   URLs/paths as single-line monospace values that would otherwise wrap. */
+.config-page .kind-editor {
+  max-width: 900px;
+}
+
 .training-type-list {
   list-style: none;
   margin: 0;