Per-mode cursor colors, configurable unfocused shape, and OSC color sync

[denieldiniz]

### Problem & Motivation

In setups where a persistent block cursor is preferred across all modes, distinguishing between Normal, Insert, and Select without repeatedly checking the statusline requires changing the cursor color.

Currently, Helix has two limitations that prevent this workflow:

1. **Cursor colors are mode-agnostic:** `[editor.cursor-shape]` in `config.toml` only controls geometry (`block`, `bar`, `underline`). Colors are defined globally in `themes/*.toml` via `ui.cursor`, with no mode-specific scopes.
2. **`FocusLost` unconditionally forces an underline:** On focus loss, Helix forces `CursorKind::Underline`. This overrides host terminal settings in modern emulators (Kitty with `cursor_shape_unfocused hollow`, Alacritty, Ghostty, WezTerm) that natively render hollow block outlines. In addition, without OSC color synchronization, the terminal falls back to its default cursor color instead of preserving the theme.

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### Proposed Solution

#### 1. Mode-specific theme scopes (`themes/*.toml`)
Add scoped fallbacks that inherit from `ui.cursor` when omitted:

```toml
"ui.cursor.normal" = { bg = "#89b4fa", fg = "#11111b" }
"ui.cursor.insert" = { bg = "#a6e3a1", fg = "#11111b" }
"ui.cursor.select" = { bg = "#f9e2af", fg = "#11111b" }
"ui.cursor.unfocused" = { bg = "#585b70", fg = "#11111b" } # optional

2. unfocused parameter in [editor.cursor-shape]

Allow configuring cursor shape behavior on focus loss:

[editor.cursor-shape]
normal = "block"
insert = "block"
select = "block"
unfocused = "match" # "match" | "block" | "bar" | "underline" | "hidden" (default: "underline")

Setting unfocused = "match" preserves the current mode's geometry on FocusLost, allowing emulators to render their native hollow block.

3. Hardware color synchronization via OSC (config.toml)

Keep the hardware cursor color aligned with the active theme:

[editor]
sync-cursor-color = true # bool (default: true)

• Dispatches OSC 12 (\x1b]12;<rgb>\x07) on mode transitions and focus loss.
• Dispatches OSC 112 (\x1b]112\x07) on application exit to restore the host terminal's default color.

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Implementation & Compatibility

• Backward compatible: Defaulting to unfocused = "underline" and falling back to ui.cursor ensures existing configurations remain unaffected.
• Backend isolation: OSC sequences and shape changes stay contained within helix_tui::Backend (crossterm/termina), keeping TestBackend clean as a no-op with no direct stdout writes from UI components.
• Zero render overhead: Escapes are dispatched strictly on mode transitions and focus events with diff checks, never on standard render ticks.

If the maintainers are aligned with this design, I am happy to open a focused PR (or separate PRs) implementing it.

[denieldiniz]

Quick update: put together a working implementation on a branch to test this out:
master...denieldiniz:helix:feature/unfocused-cursor-sync

Main changes so far:

• Added unfocused to [editor.cursor-shape] (match, block, bar, underline, hidden), defaulting to underline so nothing breaks for existing configs.
• Added sync_cursor_color flag and wired Backend::set_cursor_color (termina handles OSC 12/112 + resets on drop, crossterm is a no-op).
• Handled the focus in/out events in application.rs to sync the hardware cursor color against the theme scopes (ui.cursor.unfocused.* -> ui.cursor).

Tested locally on kitty and ghostty across mode changes and focus loss, works as expected.
Happy to squash the commits, update the docs, and open a PR if the approach looks good to you folks.