Navigate the active tab in place instead of spawning a new tab

Root cause of "settings opens new tab for every click": every
internal navigation went through `open_internal_tab → open_url →
core.open_tab(url)`, and `open_tab` unconditionally inserts a new
`BrowserTab`. So three settings sub-page clicks left four tabs in
the sidebar, which is the screenshot the user keeps sending.

Real browsers navigate the active tab in place for in-app links and
spawn new tabs only on `+ New Tab` (or Cmd-click). Wire it through:

* `BrowserTab::set_url(url)` mutates the tab's URL and bumps
  `last_active_at`. Title stays put — the page renderer can refresh
  it from the new URL.
* `BrowserCore::navigate_active_tab(url)` finds the active tab,
  calls `set_url`, marks it Ready, records the history entry, and
  bumps activity. Returns `TabNotFound` if there's no active tab.
* `ElyShell::navigate_active_tab` calls the core method and falls
  back to `open_tab` if there's no active tab to navigate. The
  shell's `open_internal_tab` (used by settings nav, home pills,
  sidebar Settings + Profile rows, command-overlay routes, etc.)
  now routes through this in-place path.
* `open_url` keeps the explicit "spawn a new tab" semantics for
  `+ New Tab` and the deep-link router.

Settings, plugin marketplace, history, profile picker — every
sidebar nav now stays in one tab.

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

crates/ely_domain/src/tab.rs

@@ -270,6 +270,16 @@ impl BrowserTab {
     pub fn set_sync_enabled(&mut self, sync_enabled: bool) {
         self.sync_enabled = sync_enabled;
     }
+
+    /// Replace this tab's URL in place. Used for in-tab navigation
+    /// (clicking a link, picking a settings sub-page, etc.) where the
+    /// active tab should follow the user instead of spawning a new
+    /// one for every URL change. Title stays as set; the caller can
+    /// re-derive it from the new URL if it wants to.
+    pub fn set_url(&mut self, url: UrlText) {
+        self.url = url;
+        self.last_active_at = SystemTime::now();
+    }
 }
 
 pub fn validate_zoom_percent(value: u16) -> Result<u16, DomainError> {