Polish sticky headers, Custom View, docs page and dialogs

- Status tables: the whole header (column names + filters) sticks as one
  block, tucked 1px under the group header, so no gap can open between
  the rows; offsets use fractional heights
- Phones/tablets: the filter panel sticks under the group header too, and
  is opaque and compact (three filters per row on phones)
- Custom View: fix the group header being pushed over the column header
  (it lives inside the view's own scroll box); the column header now pins
  to the top of that box on desktop, and the header strip sticks on phones
- All Devices: group title is "All Devices (N)" instead of "Missing
  Software", matching the page title count
- Documentation fits the window on desktop: only the article list and the
  article scroll; placeholder no longer says "from the left"
- Dialogs sit above the terminal dock so it can't cover their last rows;
  page bottom padding follows the dock's measured height

web/demo/demo.css

@@ -5,8 +5,7 @@
     font-family: 'Segoe UI', sans-serif;
     box-shadow: 0 -4px 16px rgba(0,0,0,0.35);
 }
-body { padding-bottom: 40px; }
-body.usmt-term-open { padding-bottom: calc(var(--usmt-term-h, 340px) + 72px); }
+/* Page bottom padding for the dock is set in layout.css from its measured height */
 
 .usmt-term-resize { height: 5px; cursor: ns-resize; background: transparent; }
 .usmt-term.collapsed .usmt-term-resize { display: none; }