which_key: Improve pending keystrokes indicator to include untimed chords (#64362)

Closes #64338

The pending keystrokes indicator now shows chords without a timeout,
with no countdown circle. For example, pressing `cmd-k` shows that Zed
is waiting for another key.

Few cases which needed special handling for untimed chords case:

- Hovering already pauses a timed chord, and GPUI preserves that pause
when the chord advances. An untimed chord has no paused timeout to carry
forward. We pause its newly created timeout if the pointer is still over
the indicator or popover.
- For timed chords, the timeout already stays paused during the
popover's 300 ms dismissal delay. If an untimed chord gains a timeout
during that delay, it now follows the same rule. The popover still
closes at the original deadline, then the timeout resumes with a full
second remaining.

<img width="308" height="182" alt="CleanShot 2026-09-17 at 11 45 33
AM@2x"
src="https://github.com/user-attachments/assets/9baa5fde-de9b-447d-9b02-b712a3065685"
/>

Release Notes:

- Improved the pending keystrokes indicator to show chords without a
timeout, such as shortcuts starting with `cmd-k`.

assets/settings/default.json

@@ -1973,9 +1973,9 @@
     "line_endings_button": false,
     // Control when to show the active encoding in the status bar.
     "active_encoding_button": "non_utf8",
-    // Whether to show an indicator with a countdown while timed multi-stroke
-    // input is pending. Hovering the indicator pauses the timeout. Its binding
-    // preview popover is disabled when "which_key.enabled" is true.
+    // Whether to show an indicator while multi-stroke input is pending.
+    // If the input has a timeout, a countdown is shown and hovering pauses it.
+    // Its binding preview popover is disabled when "which_key.enabled" is true.
     "pending_keystrokes_indicator": true,
   },
   // Settings specific to the terminal