fix: repair broken UX flows and dead ends across the app

[FlazeIGuess]

Fixes flow bugs and gaps found while auditing the app end to end. No release is created by this PR — the APK is uploaded as a workflow artifact under the Actions run only.

Sharing / processing

• Endless spinner fixed. Five code paths set _error without clearing _isLoading, so build kept rendering the loading state. Hitting "song not available on your service" left the user stuck forever with no way out.
• Error state gained a Try Again action; loading state gained a Cancel. The result column is now scrollable, so the alternative-service list cannot overflow on short screens.
• The pulsing logo used TweenAnimationBuilder with a fixed tween and an onEnd that called setState; the tween never changed so it pulsed once and froze. Replaced with a repeating controller.
• The metadata fast path launched the sender's platform link verbatim, ignoring the user's preferred service — the exact problem the app exists to solve. It now converts unless the link already points at the preferred service.

Playlists

• Received playlists were single-platform. The backend stores only the creator's original URL, so every imported track opened the creator's service. Links are now resolved on first open and cached back into storage.
• Import screen had no try/catch around a bare HTTP call and an error state with no actions — a failed import was an infinite spinner or a dead end. Now: timeouts, retry, and a way back.
• Re-opening the same share link created a duplicate copy each time; imports are now deduplicated by remote id.
• A failed publish was silent, dropping the user on a share screen whose share button could only fail. It now says so.
• Closing the creator discarded an in-progress playlist without asking, including via system back.
• Title/description limits were shown as errors but did not block creation.
• Tracks added from history were stored with empty convertedLinks, making them unplayable for anyone on another service.
• context.go in the create/detail/post-create flow wiped the navigation stack, so Back exited the app. Replaced with pushReplacement/pop.
• 24×24 icon-only share/QR buttons replaced with labelled 48dp targets; added pull to refresh.

History

• No way to delete a single entry. onEntryDelete was wired up but never reachable — only "Clear All" existed. Added a delete action with undo.
• Re-convert actions showed a 1–2s SnackBar for a call that can take ~30s, so the screen looked idle and repeated taps fired parallel requests. Now behind a blocking progress dialog with a re-entrancy guard.
• Re-sharing dropped the nickname that a fresh share includes, and recorded nothing, so share statistics under-reported.
• An active service filter with no matches showed "No shares yet", which reads like data loss. Now filter-aware with a "Clear filter" action.

Home / navigation / settings

• Back from any tab exited the app instead of returning Home.
• The tutorial button used context.go, replacing the stack with an onboarding flow that had no exit — users were trapped until they walked every remaining step. It is now a pushed flow with a close button.
• A Timer.periodic rebuilt the whole Home screen every 3s forever, including once the rotating placeholder was no longer displayed.
• Link field: no keyboard submit, no clear button, and the consumed link stayed behind after sharing.
• The nickname TextEditingController was constructed inside build — leaked on every rebuild and reset the caret mid-typing. Now owned by a StatefulWidget, with debounced writes.
• The version string was hardcoded; it now comes from package metadata.
• "Configure Link Handling" jumped into system settings with no explanation; the steps are shown first.
• The support dialog claimed the app is "ad-free". It is ad-supported.

Platform / plumbing

• <queries> declared no custom-scheme intents, so canLaunchUrl reported installed apps as missing on Android 11+. SMS had no <package> entry at all, so that messenger choice always silently fell back to the system share sheet.
• link_encoder.dart used print in 19 places, leaking user URLs and nicknames into release logcat.
• Playlist backend calls had no timeouts and unchecked casts.
• A return instead of continue abandoned a whole multi-item share if the first item was a deep link.
• A bare '/p/' substring test matched non-UniTune URLs; now host-scoped.

Verification

• flutter analyze — no errors, no warnings (21 remaining infos are pre-existing, mostly avoid_print in unused *_example.dart files).
• flutter test — 6/6 pass, including two new cases covering import deduplication and persisted link conversion.

Not addressed (flagged for follow-up)

• ~1,500 lines of dead code ship in the app: processing_screen_v2.dart, share_intent_handler.dart, track_search_screen.dart, batch_conversion_service.dart, cover_collage_generator.dart and the *_example.dart files are referenced from nowhere.
• Deleting a playlist keeps the remote copy alive, so its share link keeps working; a deleteToken is stored but never used.
• README advertises "3 to 10 tracks" per playlist; the code enforces no bounds at all.
• AdHelper.defaultRequest always sets nonPersonalizedAds: true, so the UMP consent flow's outcome is never applied.

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