Refactor duplicated code into shared utilities

[devin-ai-integration]

Summary

Deduplicates repeated patterns across the backend routes/services and the frontend components into shared utilities.

New backend/utils.py:

• @require_spotify_auth — replaces the if 'access_token' not in session: return 401 guard repeated in all 4 playlist routes
• get_spotify_service() — replaces the 6x repeated SpotifyService(Config.SPOTIFY_CLIENT_ID, Config.SPOTIFY_CLIENT_SECRET)
• store_token_session(token_info) — token/expiry session storage previously duplicated in callback and auth_status
• format_track(track) / format_duration(ms) — the raw-Spotify-track → frontend dict mapping (default image + m:ss duration) previously duplicated in generate_preview and Search_Track

backend/services/spotify.py:

• _token_request(data, error_label) — shared Basic-auth token POST used by exchange_code_for_token and refresh_token
• _search_request(token, query, limit, timeout) — shared search GET used by both the strict and relaxed passes of search_track (a RATE_LIMITED sentinel preserves the original 429 short-circuit vs. fall-through-on-error behavior)
• New search_tracks() and get_user_playlists() methods so Search_Track and Get_Playlists routes call the service instead of inlining raw requests calls — this also fixes a latent NameError: playlist.py used requests without importing it, so both routes would crash at runtime

Frontend (UI_Front/src/lib/playlist.ts):

• generatePlaylistName and calculateDuration were duplicated verbatim in SpotifyPlaylistGenerator.tsx and PreferenceInput.tsx; they now live in the shared module (the copies in SpotifyPlaylistGenerator were dead code and are simply removed)

.gitignore fix: lib/ → /lib/ (and lib64/). The unanchored Python-venv pattern was ignoring UI_Front/src/lib/, which is why src/lib/api.ts / src/lib/utils.ts (imported throughout the frontend) were never committed. Note: those files still need to be committed from a machine that has them — tsc currently fails with TS2307 on @/lib/api / @/lib/utils on main and this PR alike.

Verification

• python -c "from backend.app import create_app" — app builds; all routes register unchanged
• pyflakes backend/ — no new warnings (remaining unused-import warnings are preexisting)
• eslint on touched frontend files — no new errors; tsc errors are all preexisting TS2307 missing @/lib/api/@/lib/utils

Link to Devin session: https://app.devin.ai/sessions/c11c29e6d62940faa80941b4e3b536bc
Requested by: @praneels2005

[devin-ai-integration]

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