feat(overlay): add optional subtitle selection modal (#265)

changes/subtitle-selection-docs.md

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+type: docs
+area: config
+
+- Documented the subtitle selector setting, shortcut override, and primary/secondary track controls.

changes/subtitle-selection.md

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+type: added
+area: overlay
+
+- Added an optional Catppuccin subtitle selection modal for primary and secondary mpv tracks. Enable it in Settings under Behavior, then press g followed by s. Disabling it restores mpv's subtitle selection binding.
+- Single-key actions take priority over configured sequence prefixes. Conflicting sequences are disabled with a warning, and the existing y commands stay reserved.