Add shell accessibility landmarks

docs/tables_frontend_overhaul_implementation_plan.md

@@ -44,6 +44,7 @@ It is intentionally implementation-focused:
 | 2026-03-21 | P1.4 | Completed | Added shared browser-storage wrappers, persisted theme mode in `localStorage`, and initialize/apply theme state from the layout on interactive render. |
 | 2026-03-21 | P1.5 | Completed | Replaced the permanent sidebar layout with a sticky top app shell and mobile bottom navigation backed by dedicated shell components. |
 | 2026-03-21 | P1.6 | Completed | Added explicit shell slots for nav, omnibox, shortcuts, and utilities; switched shell navigation to `Play`, `Tables`, `Curation`, and `Tools`; and wired the first live theme control into the shell. |
+| 2026-03-21 | P1.7 | Completed | Added a shell-level skip link and tightened the top-level header, navigation, and main landmarks around the new shell structure. |
 
 ### Lessons Learned
 
@@ -57,6 +58,7 @@ It is intentionally implementation-focused:
 - Shared UI state events in Blazor should stay synchronous unless the event contract is async-aware. Layout refresh can be triggered safely with `InvokeAsync(StateHasChanged)` from a synchronous handler.
 - Extracting shell markup into dedicated components is lower-risk than continuing to evolve `MainLayout.razor` directly. It isolates responsive frame work from page content and keeps later nav changes localized.
 - Once a Razor component exposes multiple named `RenderFragment` parameters, the page body must be passed explicitly through `<ChildContent>`. That pattern is now the baseline for shell composition here.
+- Accessibility work is cheaper when the shell owns the landmarks. Adding skip links and nav/main structure at the shell layer avoids repeating that work page by page.
 
 ## Target Outcomes
 
@@ -264,7 +266,7 @@ Create the implementation foundation so the visual overhaul does not start with
 | `P1.4` | Completed | Theme mode now persists through a shared storage service and is applied from the layout during interactive startup. |
 | `P1.5` | Completed | The sidebar is gone; pages now render inside a sticky top-shell with a mobile bottom nav. |
 | `P1.6` | Completed | The shell now has explicit nav, omnibox, shortcut, and utility slots, plus a live theme selector and destination-model navigation. |
-| `P1.7` | Pending | Skip link and landmark work will land with the shell markup. |
+| `P1.7` | Completed | The shell now exposes a skip link and explicit header/nav/main landmarks. |
 | `P1.8` | Pending | Tools-specific shell emphasis is final Phase 1 polish. |
 
 ### Goal