fix(ui): unify Session navigation and restore uniform tab borders

DESIGN.md

@@ -165,7 +165,9 @@ The full-mode application header matches Omics Portal's `--gsd-red-primary`
 use a near-white foreground. This header is absent in embedded mode. The sidebar
 and welcome wordmarks retain their red suffix. Context editing places workspace,
 model and Done in one desktop row, stacking on narrow containers. Session-scope
-tabs retain their selected fill and accessible keyboard state without a bottom border.
+tabs retain their selected fill and accessible keyboard state with a uniform one-pixel
+border on every side, gray when inactive and red when active. Their padding is 11px
+horizontal and 3px vertical, with a 38px minimum height and wrapping labels.
 
 The palette combines warm porcelain surfaces, warm graphite text, and an instrument red used only
 for action, focus, and important state. OKLCH values in the frontmatter are normative because the
@@ -306,6 +308,9 @@ default, hover, focus, active, disabled, loading, and error behavior where those
 
 ### Navigation
 
+- **Session entry:** one Session button returns to the current unfinished session,
+  including provisional creation, without resetting or reconnecting it. Otherwise
+  it prepares a new question using the normal readiness and unsaved-work guards.
 - **Style:** compact session rows use `8px` corners and restrained vertical padding.
 - **Default / Hover / Active:** porcelain at rest, Sunken Surface on hover, and a muted Navigation
   Active red with a defined border when current. Exactly one top-level navigation control is current.
@@ -324,7 +329,9 @@ default, hover, focus, active, disabled, loading, and error behavior where those
 ### Tabs
 
 - **Shape:** compact label tabs sit on a shared baseline with rounded top corners and a two-pixel
-  lower edge, except session-scope tabs which have no bottom border. Inactive labels retain a Quiet Border and Porcelain Card surface, so every
+  lower edge, except session-scope tabs which use a uniform one-pixel border, rounded
+  corners and a 4px gap without a shared border or negative bottom margin.
+  Inactive labels retain a Quiet Border and Porcelain Card surface, so every
   label reads as a tab before interaction; hover feedback reinforces clickability.
 - **Current:** the selected tab uses the muted Navigation Active red for its fill, text, and defined border.
   It must expose `aria-selected`, participate in a labelled `tablist`/`tabpanel`, and be the only