status_bar: Add encoding indicator (#45476)

## Context / Related PRs This PR is the third part of the encoding
support improvements, following:
- #44819: Introduced initial legacy encoding support (Shift-JIS, etc.).
- #45243: Fixed UTF-16 saving behavior and improved binary detection.

## Summary
This PR implements a status bar item that displays the character
encoding of the active buffer (e.g., `UTF-8`, `Shift_JIS`). It provides
visibility into the file's encoding and indicates the presence of a Byte
Order Mark (BOM).

## Features
- **Encoding Indicator**: Displays the encoding name in the status bar.
- **BOM Support**: Appends `(BOM)` to the encoding name if a BOM is
detected (e.g., `UTF-8 (BOM)`).
- **Configuration**: The active_encoding_button setting in status_bar
accepts "enabled", "disabled", or "non_utf8". The default is "non_utf8",
which displays the indicator for all encodings except standard UTF-8
(without BOM).
- **Settings UI**: Provides a dropdown menu in the Settings UI to
control this behavior.
- **Documentation**: Updated `configuring-zed.md` and
`visual-customization.md`.

## Implementation Details
- Created `ActiveBufferEncoding` component in
`crates/encoding_selector`.
- The click handler for the button is currently a **no-op**.
Implementing the functionality to reopen files with a specific encoding
has potential implications for real-time collaboration (e.g., syncing
buffer interpretation across peers). Therefore, this PR focuses strictly
on the visualization and configuration aspects to keep the scope simple
and focused.
- Updated schema and default settings to include
`active_encoding_button`.

## Screenshots

<img width="487" height="104" alt="image"
src="https://github.com/user-attachments/assets/041f096d-ac69-4bad-ac53-20cdcb41f733"
/>
<img width="454" height="99" alt="image"
src="https://github.com/user-attachments/assets/ed76daa2-2733-484f-bb1f-4688357c035a"
/>


## Configuration
To hide the button, add the following to `settings.json`:
```json
"status_bar": {
  "active_encoding_button": "disabled"
}
```

- **enabled**: Always show the encoding.
- **disabled**: Never show the encoding.
- **non_utf8**: Shows for non-UTF-8 encodings and UTF-8 with BOM. Only
hides for standard UTF-8 (Default).

<img width="1347" height="415" alt="image"
src="https://github.com/user-attachments/assets/7f4f4938-3320-4d21-852c-53ee886d9a44"
/>

## Heuristic Limitations:
The underlying detection logic (implemented in #44819 and #45243)
prioritizes UTF-8 opening performance and does not guarantee perfect
detection for all encodings. We consider this margin of error
acceptable, similar to the behavior seen in VS Code. A future "Reopen
with Encoding" feature would serve as the primary fallback for any
misdetections.

Release Notes:

- Added a status bar item to display the active file's character encoding (e.g. `UTF-16`). This shows for non-utf8 files by default and can be configured with `{"status_bar":{"active_encoding_button":"disabled|enabled|non_utf8"}}`

crates/encoding_selector/src/active_buffer_encoding.rs

@@ -0,0 +1,91 @@
+use editor::Editor;
+use encoding_rs::{Encoding, UTF_8};
+use gpui::{
+    Context, Entity, IntoElement, ParentElement, Render, Styled, Subscription, Window, div,
+};
+use ui::{Button, ButtonCommon, Clickable, LabelSize, Tooltip};
+use workspace::{
+    StatusBarSettings, StatusItemView, Workspace,
+    item::{ItemHandle, Settings},
+};
+
+pub struct ActiveBufferEncoding {
+    active_encoding: Option<&'static Encoding>,
+    //workspace: WeakEntity<Workspace>,
+    _observe_active_editor: Option<Subscription>,
+    has_bom: bool,
+}
+
+impl ActiveBufferEncoding {
+    pub fn new(_workspace: &Workspace) -> Self {
+        Self {
+            active_encoding: None,
+            //workspace: workspace.weak_handle(),
+            _observe_active_editor: None,
+            has_bom: false,
+        }
+    }
+
+    fn update_encoding(&mut self, editor: Entity<Editor>, _: &mut Window, cx: &mut Context<Self>) {
+        self.active_encoding = None;
+
+        let editor = editor.read(cx);
+        if let Some((_, buffer, _)) = editor.active_excerpt(cx) {
+            let buffer = buffer.read(cx);
+
+            self.active_encoding = Some(buffer.encoding());
+            self.has_bom = buffer.has_bom();
+        }
+
+        cx.notify();
+    }
+}
+
+impl Render for ActiveBufferEncoding {
+    fn render(&mut self, _: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        let Some(active_encoding) = self.active_encoding else {
+            return div().hidden();
+        };
+
+        let display_option = StatusBarSettings::get_global(cx).active_encoding_button;
+        let is_utf8 = active_encoding == UTF_8;
+        if !display_option.should_show(is_utf8, self.has_bom) {
+            return div().hidden();
+        }
+
+        let mut text = active_encoding.name().to_string();
+        if self.has_bom {
+            text.push_str(" (BOM)");
+        }
+
+        div().child(
+            Button::new("change-encoding", text)
+                .label_size(LabelSize::Small)
+                .on_click(|_, _, _cx| {
+                    // No-op
+                })
+                .tooltip(Tooltip::text("Current Encoding")),
+        )
+    }
+}
+
+impl StatusItemView for ActiveBufferEncoding {
+    fn set_active_pane_item(
+        &mut self,
+        active_pane_item: Option<&dyn ItemHandle>,
+        window: &mut Window,
+        cx: &mut Context<Self>,
+    ) {
+        if let Some(editor) = active_pane_item.and_then(|item| item.downcast::<Editor>()) {
+            self._observe_active_editor =
+                Some(cx.observe_in(&editor, window, Self::update_encoding));
+            self.update_encoding(editor, window, cx);
+        } else {
+            self.active_encoding = None;
+            self.has_bom = false;
+            self._observe_active_editor = None;
+        }
+
+        cx.notify();
+    }
+}