feat: Add 'call' support to improve 2-way audio experience (#2486)

Draws significant inspiration (and direct styling) from
https://github.com/dermotduffy/advanced-camera-card/pull/2447 . Thank
you @Maudfer !

BREAKING CHANGE:

The microphone condition previously bundled two unrelated signals —
whether a two-way-audio session was connected and whether the microphone
was muted. Connection state is now its own dedicated call condition, and
microphone is reserved purely for mute state. Configs are upgraded
automatically (the card rewrites affected conditions under overrides,
elements, and automations). If you maintain config by hand, convert as
follows:

If you only used connected:

# Before
```yaml
condition: microphone
connected: true
```

# After
```yaml
condition: call
call: true
```
If you used both connected and muted — they must be split into two
conditions, since they no longer live together:

# Before
```yaml
condition: microphone
connected: true
muted: false
```

# After
```yaml
condition: and
conditions:
  - condition: call
    call: true
  - condition: microphone
    muted: false
```

src/scss/thumbnail.scss

@@ -4,8 +4,13 @@
   box-sizing: border-box;
   gap: 2px;
 
-  // Ensure control icons are relative to the thumbnail.
+  // Position control icons relative to the thumbnail, and `isolation: isolate`
+  // to keep their `z-index` scoped here. Without a stacking context an
+  // unhovered gallery tile (which has no `transform`) lets its feature icons'
+  // z-index leak into the card's stacking context and paint over the `elements`
+  // overlay.
   position: relative;
+  isolation: isolate;
   overflow: hidden;
 
   transition: transform 0.2s linear;