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/components/InlineAssistPanel.vue

@@ -188,7 +188,7 @@ const markers: Record<DiffLine["type"], string> = {
   cursor: pointer;
   font-size: 0.8rem;
   font-family: inherit;
-  font-weight: 600;
+  font-weight: var(--fs-weight-medium);
 }
 .iap-btn-accept {
   background: var(--color-success, #22c55e);

frontend/src/components/ShareDialog.vue

@@ -343,7 +343,7 @@ onMounted(async () => {
   border: none;
   border-radius: 6px;
   font-size: 0.85rem;
-  font-weight: 600;
+  font-weight: var(--fs-weight-medium);
   cursor: pointer;
   transition: opacity 0.15s;
   white-space: nowrap;

frontend/src/views/ForgotPasswordView.vue

@@ -146,7 +146,7 @@ async function handleSubmit() {
   border-radius: var(--radius-sm);
   cursor: pointer;
   font-size: 0.95rem;
-  font-weight: 600;
+  font-weight: var(--fs-weight-medium);
 }
 .btn-submit:disabled {
   opacity: 0.6;

frontend/src/views/LoginView.vue

@@ -185,7 +185,7 @@ function loginWithOAuth() {
   border-radius: var(--radius-sm);
   cursor: pointer;
   font-size: 0.95rem;
-  font-weight: 600;
+  font-weight: var(--fs-weight-medium);
 }
 .btn-submit:disabled {
   opacity: 0.6;
@@ -217,7 +217,7 @@ function loginWithOAuth() {
   border-radius: var(--radius-sm);
   cursor: pointer;
   font-size: 0.95rem;
-  font-weight: 600;
+  font-weight: var(--fs-weight-medium);
 }
 .btn-oauth:hover {
   background: var(--color-bg-hover, var(--color-border));

frontend/src/views/RegisterInviteView.vue

@@ -256,7 +256,7 @@ async function handleSubmit() {
   border-radius: var(--radius-sm);
   cursor: pointer;
   font-size: 0.95rem;
-  font-weight: 600;
+  font-weight: var(--fs-weight-medium);
 }
 .btn-submit:disabled {
   opacity: 0.6;

frontend/src/views/RegisterView.vue

@@ -225,7 +225,7 @@ async function handleSubmit() {
   border-radius: var(--radius-sm);
   cursor: pointer;
   font-size: 0.95rem;
-  font-weight: 600;
+  font-weight: var(--fs-weight-medium);
 }
 .btn-submit:disabled {
   opacity: 0.6;

frontend/src/views/ResetPasswordView.vue

@@ -204,7 +204,7 @@ async function handleSubmit() {
   border-radius: var(--radius-sm);
   cursor: pointer;
   font-size: 0.95rem;
-  font-weight: 600;
+  font-weight: var(--fs-weight-medium);
 }
 .btn-submit:disabled {
   opacity: 0.6;

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) {