fix(subtitles): remove the subtitle outline when it is disabled

[RakshithBhat03]

Summary

With custom subtitle styling enabled, the macOS and Windows desktop bridges pushed the subtitle outline as sub-outline-size / sub-outline-color. The bundled mpv revision (c0dd2b32d52c66b9efa85a7ee69965569503f6b8, the revision documented in native/macos/mpv/README.md) does not define either property, so both writes were rejected silently: mpv kept drawing its default outline, Outline: Off changed nothing, and the outline color swatches were dead for the same reason.

The bridges now write sub-border-size / sub-border-color, which exist in every mpv revision — newer builds alias --sub-outline-* to them. The iOS bridge in this repository carried the same two names and is fixed in the same commit.

PR type

• Reproducible bug fix
• UI glitch/bug fix
• Behavior bug/regression fix
• Small maintenance only, with no UI or behavior change
• Docs accuracy fix
• Translation/localization only
• Approved larger or directional change

Why

The subtitle outline toggle and outline color had no effect whenever custom styling was on. The bundled runtime rejects sub-outline-size and sub-outline-color with property not found, so the player never applied the user's setting and always rendered mpv's default outline. Writing --sub-border-* makes the size and color apply on the bundled revision while staying valid on newer mpv builds.

Desktop scope

• macOS: composeApp/src/desktopMain/native/macos/player_bridge.mm
• Windows: composeApp/src/desktopMain/native/windows/player_bridge.cpp

Linux desktop already used --sub-border-* and is unchanged. The same property names in iosApp/iosApp/Player/MPVPlayerBridge.swift are fixed in this commit because that bridge lives in this repository.

Issue or approval

Root cause of NuvioMedia#593. Fork-only PR, based on the enhanced branch of this fork; nothing is submitted upstream from here.

UI / behavior impact

• No UI change
• No behavior change
• UI changed only to fix a documented glitch/bug
• Behavior changed only to fix a documented bug/regression
• UI change has explicit maintainer approval
• Behavior change has explicit maintainer approval

Policy check

• I have read and understood CONTRIBUTING.md.
• This PR is small, focused, and limited to one problem.
• This PR is scoped to the desktop app, desktop packaging, desktop documentation, or shared code required for desktop behavior.
• This PR is not cosmetic-only.
• Any UI change fixes a linked glitch/bug and includes visual proof, or this PR has no UI change.
• Any behavior change fixes a linked bug/regression or has explicit approval, or this PR has no behavior change.
• This PR does not bundle unrelated refactors, cleanups, formatting, or drive-by changes.
• This PR does not add dependencies, architecture changes, migrations, or product-direction changes without explicit approval.
• I listed the testing performed below.

Scope boundaries

• The bundled mpv runtime is untouched: same revision, same dependency dylibs, no version or provenance change.
• No UI change. The outline width control proposed upstream (Fix macOS subtitle outline toggle and add outline width NuvioMedia/NuvioDesktop#444) is not part of this PR, so the desktop keeps sending the stored width.
• Linux and Android bridges unchanged — they already set the names this runtime accepts.
• sub-border-style is left as written: it is a no-op on the bundled revision, where outline-vs-box follows --sub-back-color alpha, and it is required on newer mpv builds.
• Settings storage, wire contracts, and packaging are unchanged.

Testing

• Runtime probe of the bundled macOS libmpv.2.dylib on arm64 and on x86_64 under Rosetta (mpv-version reads back mpv c0dd2b32d52c66b9efa85a7ee69965569503f6b8-dirty): sub-outline-size / sub-outline-color → MPV_ERROR_PROPERTY_NOT_FOUND; sub-border-size / sub-border-color → accepted.
• Offscreen OpenGL render of an ffmpeg-generated MKV with an ASS track (1280x720, sub-ass-override=force, red outline on black video, red-pixel count): sub-outline-size=0 → 8921 outline pixels (outline still drawn), sub-border-size=0 → 0 outline pixels, sub-border-size=2 → 6163 outline pixels. Identical results on the x86_64 runtime under Rosetta.
• macOS bridge compiled with clang++ -fsyntax-only against the bundled headers, then ./gradlew :composeApp:packageReleaseDmg --rerun-tasks on macOS 26.6.2 (Apple Silicon) succeeded. The packaged libplayer_bridge.dylib inside the DMG contains sub-border-size / sub-border-color and no sub-outline-*.
• Not run: the Windows bridge (the bundled libmpv-2.dll is an unmaterialized Git LFS pointer in this checkout, and there is no Windows host available) and the iOS bridge (no MPVKit/Xcode build). Both are the same one-line rename to property names that every mpv revision accepts.
• The DMG built for this check is ad-hoc signed — no Developer ID identity is available in the keychain — and was not notarized, so no signed-release verification is claimed.

Screenshots / Video

Not a UI change.

Breaking changes

None.

Linked issues

Fixes NuvioMedia#593.