refactor(ui): buttons stop using weights the system doesn't have

Eleven buttons set font-weight 600. The house style permits exactly two
weights, 400 and 500, and says so explicitly — 600 and 700 are not part of the
system. Every auth Submit, plus Invite, Toggle, Confirm-delete, Add-share,
the OAuth button and the assist Reject.

Now var(--fs-weight-medium), which is 500. Buttons get very slightly lighter.

Small on its own, but it is the third kind of drift the same five auth views
have now produced: geometry that differed per file, an accent fill the style
forbids, and a weight the system does not define. None of the three was a
deliberate choice — each is what happens when a button is written by copying
the nearest existing one.

Weight declarations only. No geometry, no colour, no templates.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

frontend/src/views/UserManagementView.vue

@@ -336,7 +336,7 @@ function formatDate(iso: string): string {
   border-radius: var(--radius-sm);
   cursor: pointer;
   font-size: 0.9rem;
-  font-weight: 600;
+  font-weight: var(--fs-weight-medium);
   white-space: nowrap;
 }
 .btn-invite:disabled {
@@ -386,7 +386,7 @@ function formatDate(iso: string): string {
   border-radius: var(--radius-sm);
   cursor: pointer;
   font-size: 0.9rem;
-  font-weight: 600;
+  font-weight: var(--fs-weight-medium);
   white-space: nowrap;
 }
 .btn-toggle:disabled {
@@ -503,7 +503,7 @@ function formatDate(iso: string): string {
   border-radius: var(--radius-sm);
   cursor: pointer;
   font-size: 0.8rem;
-  font-weight: 600;
+  font-weight: var(--fs-weight-medium);
   margin-right: 0.25rem;
 }
 .btn-confirm-delete:hover:not(:disabled) {