Plan Flutter web support

[SamDozer]

Exploration, not a commitment.

The architecture is favourable: practice_core is pure Dart with no Flutter
dependency, and the UI reads theme tokens rather than platform APIs.

What would need work:

• Database. drift supports web via WASM, but the current setup is
  native-only.
• Audio. The metronome is a Kotlin AudioTrack engine behind
  practice_audio_api. Web would need a second implementation — Web Audio,
  probably — behind the same contract. The contract exists precisely so this is
  possible.
• Background cues. No equivalent of AlarmManager. Probably out of scope.
• Layout. Every screen is phone-shaped today.
• Data movement. Import/export between Android and web, which is another
  reason Design the routine export/import format #3 comes first.

Deliverable: a written assessment of effort and of what would have to be
dropped.