feat(app): add dashboard showcase tour

[mohiuddinshahrukh]

Fixes #799

Summary

• Add showcaseview to the app package.
• Add a one-time dashboard tour for new users covering study progress, current intervention, today's tasks, contact, reports, and the overflow menu.
• Store completion in secure storage so the tour does not replay after refresh/reopen.
• Add localized English and German showcase text.

Tested

• flutter gen-l10n
• dart format on the touched app files
• flutter analyze
• Manual frontend check on the dashboard: the showcase appears, can be dismissed/completed, and does not reappear after refresh.

[github-actions]

Visit the preview URL for this PR (updated for commit 9ffe9aa):

• https://studyu-dev-app--pr828-feat-799-showcasevie-kulmw8l8.web.app
• https://studyu-dev-designer--pr828-feat-799-showcasevie-aht9u2mz.web.app

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[ibrahimozkn]

Could you include a video of the dashboard?