fix: repeat-mode infinite loop and state desync; feat: word-highlight and repeat-phrase (issue #232)

[nmwael]

WHY

Issue #232's language-learning features (word highlight + repeat phrase) exposed two bugs in PlayerNotifier:

1. The playback-completion listener guarded currentIndex < entries.length, which is always true for a 0-based index, so repeat mode loops the last subtitle forever.
2. isPlaying / isPaused were never updated when a subtitle repeated, desyncing the UI state.

WHAT

Word-highlight and repeat-phrase language-learning features plus repeat-mode fixes.

• Fix 1: the completion listener now compares currentIndex < state.entries.length - 1.
• Fix 2: any repeat (auto or manual) sets isPlaying: true, isPaused: false and delegates to a shared private _repeatCurrent().
• New unit / widget / BDD tests and regenerated goldens.
• opencode subagent config (per-agent LSP permission, provider models maps), rebased cleanly onto the latest development while keeping PR chore: spread agent model assignments across providers to reduce rate limiting #231's provider-spread model assignments.

HOW

Follows TDD with BDD: the tester first wrote failing red tests (player_notifier_repeat_mode_test.dart with a fake TtsRepository exposing a repeat-call counter, plus the existing language-learning spec tests), then the developer made them green.

• Full suite validated: unit 192, widget 23, golden ~19, BDD features 14 — all green.
• dart analyze and dart format clean.
• Security audit: PASS.
• Branch synced with development (PR chore: spread agent model assignments across providers to reduce rate limiting #231) via rebase; GPG-signed commits.