Unify snackbars behind a coalescing queue and a shared theme

[ultraelectronica]

Summary

Replaces ad-hoc ScaffoldMessenger.showSnackBar calls with one app-wide
snackbar surface: consistent styling, deduplication, queueing, and a
lifetime that respects the user's motion, accessibility, and app state.

Shared API and theme

• Adds lib/widgets/common/flick_snackbar.dart: an extension on ScaffoldMessengerState with showFlickSnackBar(SnackBar, {Object? id}), hideFlickSnackBar({Object? id}), and clearFlickSnackBars(). Each messenger gets its own queue via an Expando, so state does not leak between hosts or tests.
• Adds a shared SnackBarTheme (floating, surfaceLight surface, glass border, radiusMd, zero elevation, body text style, accent action text, close icon, 16/8/16/12 inset) to AppTheme.darkTheme.
• Migrates every direct ScaffoldMessenger.showSnackBar call across 45 files (player sheets/controls/navigation, lyrics, playlists, favorites, settings, duplicate cleaner, logs, UAC2 prompts, ALAC conversion, and more). No direct calls remain in lib/.

Queue, coalescing, and identity

• Routine notices (no action, not persistent, under one minute) are coalesced: an identical id + text + background while visible only refreshes the timer, and a rapid burst keeps only the latest routine message instead of stacking.
• Actionable and long-running notices are protected: they queue behind each other and are never evicted by routine feedback. A note with a matching id may update itself without replaying a stale backlog.
• id enables targeted dismissal. playback-desync and metadata-warmup in lib/app/app.dart are now hidden by id, so clearing one status no longer wipes unrelated Undo/Retry feedback.
• The custom Undo row in songs_screen.dart becomes a real SnackBarAction, so it is protected and keeps its callback while routine toasts come and go.

Motion, accessibility, and lifetime

• Keeps native placement, swipe dismissal, live-region semantics, and action handling, but animates the whole surface: fade + 12px rise over 280ms with easeOutCubic, exit over 200ms, instead of Material 3's narrow fade interval. Reduced motion or the app's zero-animation preference uses AnimationStyle.noAnimation.
• Each notice owns a mounted-content timer rather than an app-wide poll. It pauses while the app is backgrounded, hovered, or focused, resumes when reading stops, and gives screen-reader users at least 8 seconds. Timers and pending notices are cancelled when the host is disposed, and the same notice can be mounted in two routes during a Hero transition without a duplicate GlobalKey.

Player bar clearance

• MainShell wraps its subtree in a SnackBarTheme whose bottom insetPadding adds the nested bar clearance (150 + extraClearance + 12). The player/navigation bar is a body overlay, so Scaffold cannot infer that reserved space on its own.

Tests

• Adds 15 widget tests in test/widgets/flick_snackbar_test.dart covering the 280/200ms surface animation, replacement during entry without an opaque flash, duplicate-timer refresh, burst coalescing, Undo survival and callback integrity, protected queueing, targeted dismissal, named-status replacement, swipe-to-advance, reduced motion, background pause/resume, screen-reader duration and semantics, keyboard/RTL/2x-text layout, route transitions, and dispose cleanup.

Related issue

none

Checklist

• flutter analyze passes
• flutter test passes
• Rust changes: cargo test --manifest-path rust/Cargo.toml passes
• Built and ran the app if the change touches a playback path (flutter run --flavor full)
• Generated code regenerated and committed if bridge/drift/freezed models changed
• CHANGELOG.md updated for user-facing changes
• No secrets, keys, tokens, or machine-local files committed
• One change per PR; no unrelated reformatting in the same diff
• Hardware tested if the Rust audio engine or USB/DAC output changed