Deliver localized auth app with verification and go_router

Integrate themed login/signup, EN/RU localization, shared dummy providers, verification recovery screens and named URL routes. Store frontend guidelines, workflow skills, validation and delivery documentation. Related to #2, #3, #5 and #6.

.agents/skills/review-frontend-change/SKILL.md

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+---
+name: review-frontend-change
+description: Review, fix, and validate the complete Sendico frontend contribution for a task. Use before reporting completion whenever Dart or Flutter code under frontend/ was added or modified, or when the user requests a frontend implementation review.
+---
+
+# Review Frontend Change
+
+## Establish the review scope
+
+1. Read the repository root `AGENTS.md` and `frontend/AGENTS.md` completely.
+2. Inspect repository status and the full frontend diff attributable to the
+   current task, including uncommitted and committed changes against the task's
+   base.
+3. Separate unrelated pre-existing work from the task contribution. Preserve
+   it and do not include it in review fixes.
+4. Identify every changed Dart file and each affected Flutter package before
+   validation.
+
+## Review and fix the contribution
+
+1. Apply every quality requirement in `frontend/AGENTS.md` to the entire task
+   diff, not only the most recently edited file.
+2. Verify the established DTO-to-mapper-to-service-to-model-to-provider flow and
+   ensure widgets do not absorb transport, I/O, or state-orchestration work.
+3. Search for reusable DTOs, models, providers, widgets, and utilities before
+   accepting new representations.
+4. Review widget structure, file cohesion, lifecycle behavior, asynchronous
+   races, notification scope, accessibility, responsive behavior, and import
+   header layout.
+5. Review loading, empty, success, and error states together with nullability,
+   navigation, user-visible failures, edge cases, and focused test coverage.
+6. Fix every in-scope issue found, then re-read the resulting complete frontend
+   diff. Repeat until no task-caused finding remains.
+
+Do not weaken lints, suppress failures, or modify unrelated user work to make
+validation pass.
+
+## Validate
+
+1. Run `dart format --output=none --set-exit-if-changed` for every changed Dart
+   file.
+2. Run `flutter analyze` from every affected frontend package directory.
+3. Run `make test-frontend` from the repository root.
+4. Select and run every applicable security command from the change-scoped
+   security completion requirements in the root `AGENTS.md`.
+
+All commands must succeed before reporting the frontend task as complete. If a
+command fails, determine whether the task caused it and fix in-scope failures.
+Report an external or environmental blocker with the exact command and failure;
+never represent blocked validation as successful.
+
+## Report the result
+
+Summarize the reviewed frontend scope, material fixes made during review, tests
+added or updated, affected packages, and the outcome of every required command.
+Call out relevant accessibility, behavior, contract, or rollout implications.