Track web target support strategy for bdk-dart

Define whether and how bdk-dart can support web usage, and what level of support is realistic in the near term.

## Scope

- Evaluate technical feasibility of web support with current bdk-ffi/UniFFI/native-assets model.
- Document required architecture options:
- wasm-based path
- service/backend-assisted path
- “not currently feasible” with clear rationale
- Identify security and wallet-model implications for browser environments.
- Define expected user experience and limitations.
- Decide support level for web in SUPPORTED_TARGETS.md (supported, experimental, not supported).

## Deliverables

- [ ] A short design note (docs/web-support.md) with recommendation and tradeoffs.
- [ ] Explicit web status in SUPPORTED_TARGETS.md.
- [ ] README note linking to the design/status.
- [ ] Follow-up implementation issues (if feasible path is chosen).

## Acceptance Criteria

- [ ] Maintainers align on a documented recommendation.
- [ ] Users can clearly understand whether web is currently supported.
- [ ] If not supported yet, blockers and next steps are explicit.
- [ ] #16 checklist links this issue.

## Notes

- This issue is about strategy and scope definition first.
- Implementation can be split into follow-up issues once direction is agreed.