feat: add 8 new features

[ivo9999]

Summary

• Interview History: /history page with localStorage token tracking, Flyway V4 migration for user_token, topic_focus, interview_length columns
• Practice Mode with Topic Focus: chip selector in setup step 2 (general, system design, behavioral, algorithms, culture fit) integrated into interviewer prompts
• Live Transcript Panel: real-time sliding panel during interview with chat-bubble rendering using safe DOM API
• Interview Replay: collapsible transcript section in report page with speaker-labeled entries
• Shareable Report Links: copy link button using clipboard API
• More Languages (DE/ES/FR): full i18n message files, flag SVGs in language dropdown, prompt and grading language support
• Custom Interview Length: quick (~3Q), standard (~5-7Q), marathon (~10+Q) selector with dynamic prompt question counts
• Dark/Light Theme Toggle: CSS class-based light theme overrides, localStorage persistence, flash prevention

Files changed

• 54 files changed, ~2470 insertions, ~226 deletions
• 6 new files: HistoryController.java, history.html, V4 migration, messages_de/es/fr.properties

Test plan

• Verify history page loads at /history and shows past sessions
• Verify topic focus chips appear in setup step 2 and affect interview prompts
• Verify live transcript panel toggles during interview and shows messages
• Verify transcript replay section appears in report page
• Verify copy link button copies report URL to clipboard
• Verify DE/ES/FR language switching works end-to-end (UI, prompts, grading)
• Verify interview length selector affects question count in prompts
• Verify dark/light theme toggle persists across page navigation
• Verify V4 migration runs cleanly on existing database

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