Add feature-gated dual subtitle MVP with merged subtitle track

[beng1z]

Summary

• Add a feature-gated dual subtitle MVP (off by default).
• Add explicit Primary / Secondary selection mode in source/subtitle dialog.
• Implement parse-time subtitle merge into a single generated WebVTT track (no polling).
• Block unsupported combinations (embedded secondary, embedded primary+secondary, secondary without primary) with clear user feedback.
• Keep 403/header bugfix out of this PR scope, except dual-pipeline-required subtitle header merge.

What changed

• IPlayer API expanded with secondary subtitle methods.
• CS3IPlayer implements secondary state, dual merge pipeline, and merged track selection.
• GeneratorPlayer updated for Primary/Secondary selection UX and Apply-based commit behavior.
• Subtitle settings now include Dual subtitles (experimental) switch.
• Added DualSubtitleComposer + unit tests.
• Added/updated localized strings for all supported locales for the newly introduced dual subtitle keys.

AI usage disclosure

• AI tooling was used to assist implementation and localization generation.
• All code/resources were manually reviewed and adjusted.
• Review feedback and requested UX constraints were applied manually.

Testing

• Manual device test completed (dual subtitle selection, apply flow, unsupported combo blocking, layout behavior).
• ./gradlew testStableDebugUnitTest assembleStableDebug --no-daemon passes.

Notes

• Dual subtitles remain experimental and disabled by default.
• Embedded-secondary scenarios are intentionally unsupported in MVP.

[beng1z]

Follow-up update:

• Rebased the branch onto current master and squashed the five commits into one so the diff now reflects only the dual-subtitle changes (11 files, no locale churn).
• Fixed the main-thread freeze that caused the error loop you observed: the merge pipeline in CS3IPlayer.buildMergedDualSubtitleSource previously did synchronous HTTP reads inside getSubSources, which is called from loadOnlinePlayer on the UI thread. loadSubtitleBytes now runs inside runBlocking(Dispatchers.IO) and the merged .vtt is reused from the cache on reload, so no re-download/re-parse on the retry path.
• Cleaned up leftover cosmetics in values/strings.xml (restored blank_repo_message whitespace, fixed indentation, dropped the now-unused subtitles_secondary_load_failed string).
• ./gradlew testStableDebugUnitTest assembleStableDebug both pass locally.

I understand the maintainability concern about merging an MVP — if there are specific polish items beyond what's above that need to land before this is acceptable, I'm happy to address them in-place rather than as a follow-up.