fix(ChannelList): make channel list and thread list width app-driven (#3235)

### 🎯 Goal

The SDK shouldn't hold strict opinions about how wide the channel list
(and thread list) sidebars are — that's a layout decision the parent app
owns. Previously `--str-chat__channel-list-width` defaulted to `30%`,
effectively dictating a desktop two-pane ratio out of the box. This
makes the sidebar width app-driven: the SDK defaults to filling
available space, and the app constrains it via the CSS variable (as the
vite example already does with its own layout token).

### 🛠 Implementation details

- **`ChannelList.scss`** — change the default
`--str-chat__channel-list-width` from `30%` → `100%`. The channel list
is `flex: 0 0 var(--str-chat__channel-list-width)`, so apps drive the
width by overriding the variable (not the `width` property, since a
non-`auto` `flex-basis` wins in a flex row).
- **`ThreadList.scss`** — decouple `--str-chat__thread-list-width` and
`--str-chat__thread-list-mobile-width` from the channel-list tokens.
They previously resolved to `var(--str-chat__channel-list-width)` /
`var(--str-chat__channel-list-mobile-width)`, which meant sizing the
channel list silently resized the thread list too. They now have their
own independent `100%` defaults, so each sidebar is sized independently.
- Transition tokens (`duration`/`easing`/`offset`) remain shared — those
are animation consistency, not width, and are unchanged.

**Notes / compatibility**

- Behavioral default change: an app relying on the old built-in `30%`
desktop sidebar will now see a full-width channel list until it sets
`--str-chat__channel-list-width` itself.
- Mobile is unaffected — the mobile breakpoint already forces a `100%`
absolutely-positioned overlay with `min-width: 0`.
- Examples that already override the variable (`examples/vite`, tutorial
shell) are unaffected. `yarn build-styling` compiles cleanly.

### 🎨 UI Changes

No screenshots attached — this is a CSS custom-property default change
with no markup changes. Visible effect: without an app-provided width
override, the desktop channel list renders full-width (fills its flex
container) instead of the previous `30%` sidebar. Apps set
`--str-chat__channel-list-width` (and now optionally
`--str-chat__thread-list-width` independently) to restore a
fixed/percentage sidebar.

Author: oliverlaz

src/components/ChannelList/styling/ChannelList.scss

@@ -11,7 +11,7 @@
   --str-chat__channel-list-transition-offset: 8px;
 
   /* Default desktop width of the channel list. Override to match custom layouts. */
-  --str-chat__channel-list-width: 30%;
+  --str-chat__channel-list-width: 100%;
 
   /* Default mobile overlay width of the channel list. Override to match custom layouts. */
   --str-chat__channel-list-mobile-width: 100%;

src/components/Threads/ThreadList/styling/ThreadList.scss

@@ -10,8 +10,11 @@
   --str-chat__thread-list-transition-offset: var(
     --str-chat__channel-list-transition-offset
   );
-  --str-chat__thread-list-width: var(--str-chat__channel-list-width);
-  --str-chat__thread-list-mobile-width: var(--str-chat__channel-list-mobile-width);
+  /* Default desktop width of the thread list. Override to match custom layouts. */
+  --str-chat__thread-list-width: 100%;
+
+  /* Default mobile overlay width of the thread list. Override to match custom layouts. */
+  --str-chat__thread-list-mobile-width: 100%;
 }
 
 .str-chat__thread-list-container {