feat(ui): draw the open right rail tab and its panel as one surface

The strip traces the joined silhouette of the open panel and its tab as an SVG path (rounded corners, concave fillets; Cyber's chamfered frame and a square tab) and keeps a join element under the panels: one layer blurs and tints the backdrop inside the path, an SVG strokes the outline along it and another casts the drop shadow. The panel card and the tab draw no surface of their own, so the junction has no seam, doubled edge or second blur region. Cyber's generic selected-button treatment is kept off the tab.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

docs/CURRENT-STATE.md

@@ -1509,20 +1509,20 @@ the panels' own translated header titles. The strip marks the rail with
 rail without the strip keeps its legacy geometry. Phones hide the strip and
 keep the sheet shell; Cockpit hides the rail and the strip with it.
 
-The open tab and its panel read as one shape, like a folder tab: they share
-the glass surface and one outline, and the other tabs are borderless labels.
-The panel card keeps its surface but draws no border; the rail's `::before`
-draws the outline around the card with a notch where the tab joins, the open
-tab closes the shape with its own top, right and bottom borders, and two
-concave fillets round the inner corners (Cyber's chamfered frames join
-square, and its strip starts below the frame's upper chamfer). The strip
-publishes the join on the rail as inline custom properties, re-measured
-through a ResizeObserver and on strip scroll: `--rail-tab-top` and
-`--rail-tab-bottom` place the notch and set the open panel's minimum height,
-so a short panel such as Context still reaches its tab, and
-`--rail-panel-height` sizes the outline, since the rail is taller than its
-panel in the focus layout. The first tab starts level with the panel's top
-edge, so the outline's top runs straight into it with no corner.
+The open tab and its panel are one surface, like a folder tab, and the other
+tabs are borderless labels. The strip measures the open panel and tab,
+traces their joined silhouette as an SVG path (`railJoinPath`: rounded panel
+corners, the tab's outer corners, concave fillets where the tab meets the
+panel's edge; Cyber's chamfered frame with its calibration bay and a square
+tab instead) and keeps a join element under the panels: one layer blurs and
+tints the backdrop inside the path, an SVG strokes the outline along it and
+another SVG casts the drop shadow, so the junction has no seam, doubled edge
+or second blur region. The open panel's card and the tab draw no surface,
+border, shadow or blur of their own. The join is re-measured through a
+ResizeObserver and on strip scroll; `--rail-tab-bottom` on the rail sets the
+open panel's minimum height, so a short panel such as Context still reaches
+its tab. A first tab level with the panel's top shares its top edge, with no
+corner between them; Cyber's strip starts below the frame's upper chamfer.
 
 ## Phone layout