Add shared table context state

docs/tables_frontend_overhaul_implementation_plan.md

@@ -55,6 +55,7 @@ It is intentionally implementation-focused:
 | 2026-03-21 | P2.3 | Completed | Added a shared `RecentTablesState` service backed by browser storage, centralized the recents storage key, and started recording successful table visits from the `Tables` page so later omnibox and rail work has real shared data. |
 | 2026-03-21 | P2.4 | Completed | Added a shared `PinnedTablesState` service with browser persistence, centralized the pin storage key, initialized pin state in the `Tables` page, and added a first live pin/unpin action plus pinned status chips so later omnibox and navigation work have real saved pins to consume. |
 | 2026-03-21 | P2.5 | Completed | Added shared table-context URL types and a serializer, registered the serializer centrally, and started round-tripping table context through `/tables` and `/tools/diagnostics` so direct links restore the selected table and diagnostic cell context instead of relying only on local storage defaults. |
+| 2026-03-21 | P2.8 | Completed | Added a shared `TableContextState` service on top of browser storage and the URL serializer, moved the `Tables` page off page-local table selection persistence, and switched diagnostics to the same restore/persist/build-URI flow so table context logic now lives in shared frontend state instead of being reinvented per page. |
 
 ### Lessons Learned
 
@@ -79,6 +80,7 @@ It is intentionally implementation-focused:
 - Shared browser-backed state becomes more useful once one real page writes to it immediately. Recording recents from `Tables` now keeps later omnibox work from being blocked on synthetic placeholder data.
 - Shared pinned-state services also need one live writer early. A minimal pin/unpin affordance in the current `Tables` page is enough to validate persistence before the larger navigation surfaces consume pins.
 - URL serializers only pay off when wired into real pages. Using the shared serializer in both `Tables` and diagnostics now gives the project one verified round-trip path before the larger table-context service lands.
+- The serializer alone is not the reusable boundary. The maintainable seam is a shared context-state helper that owns restore, persist, normalization, and URI-building conventions while pages keep only workflow-specific selection logic.
 
 ## Target Outcomes
 
@@ -373,7 +375,7 @@ Establish the shared shell, tokens, typography, and theme system that every dest
 | `P2.5` | Completed | Shared table-context URLs now parse and serialize through one helper and are consumed by `/tables` and `/tools/diagnostics`. |
 | `P2.6` | Pending | The shell omnibox is still a placeholder trigger. |
 | `P2.7` | Pending | Shared primitives for chips, tabs, drawers, and inspector sections are not extracted yet. |
-| `P2.8` | Pending | Table-selection logic still lives inside individual pages. |
+| `P2.8` | Completed | Table-context restore, persistence, normalization, and URI building now flow through one shared state service used by `Tables` and diagnostics. |
 
 ### Goal