feat: Active/Completed tabs on My Log page (v3.4.0)

Replaces the two stacked sections (Active above, Completed below)
on the main Logbook page with a single-pane WP-admin tabbed view
using the native subsubsub pattern (same as Posts/Comments/Plugins).

- Two tabs: Active (N) | Completed (M) with live counts
- URL-driven state via ?tab=active|completed; bookmarkable,
  refresh-stable, back-button works, invalid values fall back to
  Active server-side via whitelist + sanitize_key
- No JavaScript — each tab is a hyperlink built with add_query_arg
- WP-admin core CSS handles .subsubsub + .current; only a small
  margin block added in wp-notes-styles.php
- Single-pane render: only the selected tab's section is in the
  DOM, so no flash-of-wrong-content
- "Add a Note" form stays visible on both tabs for UI consistency

Flagged in the 2026-05-25 UX audit as the highest-payoff next move
(Tier 3 item #7). When the v4 timer/time_logged field ships, the
structure can extend to 3 tabs (TODO/IN PROGRESS/Completed) in
the same place; without it, an IN PROGRESS tab would always be
empty, so v3.4.0 ships the 2-tab version matching the current
data model.

Storage model unchanged — Active and Completed remain in their
separate options (wp_notes, wp_done_notes). The
wp_notes_display_notes($type) function is unchanged.

MINOR bump: new feature, no breaking changes.

inc/wp-notes-styles.php

@@ -198,6 +198,14 @@ function wp_notes_admin_styles() {
             color: #3c763d;
         }
 
+        /* v3.4.0 — Active/Completed tab strip on the My Log page.
+           WP-admin core already styles .subsubsub and .current;
+           we only need to add breathing room above + below. */
+        .wp-notes-page .subsubsub,
+        .wrap > .subsubsub {
+            margin: 12px 0 18px;
+        }
+
         /* Responsive Design */
         @media screen and (max-width: 782px) {
             .wp-notes-formatting {