Omnibar Enter navigates the current tab instead of spawning a new one

`submit_command` matched every browser's "Enter to navigate" intent
to `open_tab(url)`, which always inserts a new `BrowserTab`. So
typing `google.com` and hitting Enter on a new-tab page would leave
both the new-tab and a fresh google.com tab in the sidebar.

Mirror the same in-place navigation rule the shell-level code now
uses: `navigate_active_tab(url)` for the Navigate and Search
intents, with `open_tab` as the fallback when there's no active tab
yet. Tab count only goes up when the user explicitly hits + New Tab.

cargo test --workspace: 440 passed, 0 failed.

crates/ely_browser_core/src/state/commands.rs

@@ -29,12 +29,20 @@ impl BrowserCore {
         let intent = CommandIntent::parse(query)?;
         match &intent {
             CommandIntent::Navigate(url) => {
+                // Submitting a URL navigates the current tab in place,
+                // matching every real browser. If there's no active tab
+                // to navigate (no shell yet, no tabs at all), fall
+                // back to spawning a new one.
+                if self.navigate_active_tab(url.clone()).is_err() {
                     self.open_tab(url.clone());
+                }
                 self.command_query.clear();
             }
             CommandIntent::Search(query) => {
                 let url = search_url(query, self.search_engine)?;
+                if self.navigate_active_tab(url.clone()).is_err() {
                     self.open_tab(url);
+                }
                 self.command_query.clear();
             }
             CommandIntent::Command(command) if self.submit_named_command(command)? => {