Task/Issue URL:
https://app.asana.com/1/137249556945/project/1212810093780571/task/1215364312660003?focus=true

### Description

Fixes the issue where the app **exits** when you open full-screen
Duck.ai (from the browser or duck.ai entry point) and then close it (the
X in Duck.ai, or the back button). Mainly visible from **New Voice
Chat** in the menu, but it affects every entry that opens Duck.ai via
the `openDuckChat` intent.

The handler now anchors the new tab to either an explicitly passed
`SOURCE_TAB_ID_EXTRA` if present, or the current tab as a fallback:

### Known limitation

Opening from the **New Tab Page** still exits the app. Will address as a
follow up if needed.

### Steps to test this PR
- [ ] From an open tab (a page or Duck.ai), open the menu → **New Voice
Chat**.
- [ ] Press the **X** (top-left in Duck.ai) → returns to the tab you
were on (does **not** exit the app).
- [ ] Repeat and press the **back button** → also returns to the
previous tab.
- [ ] Sanity: the omnibar Duck.ai button / other Duck.ai opens from a
tab also return on close.

### UI changes

No visual UI changes



<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Small, localized navigation change in intent handling; reuses existing
source-tab plumbing with no auth or data changes.
> 
> **Overview**
> Fixes **full-screen Duck.ai** closing the app instead of returning to
the tab you came from (e.g. **New Voice Chat** from the menu).
> 
> When handling `OPEN_DUCK_CHAT` with full-screen mode enabled, the new
Duck.ai tab is now opened with a **`sourceTabId`**: from
`SOURCE_TAB_ID_EXTRA` on the intent when present, otherwise
**`currentTab?.tabId`**. That value is passed through both the
swiping-tabs (`launchNewTab`) and legacy (`onOpenInNewTabRequested`)
paths so closing the Duck.ai tab can restore the prior tab.
> 
> Opening from the **New Tab Page** without a current tab may still exit
the app; called out as a known follow-up.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
d2c7b0fcfb54b8953d2da89a28ba504acc688317. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->