Add web app build and hosting configuration

[Salint]

This PR introduces a web version of the TwoAxis Finance app. It configures the Flutter project for web compilation, updates the Firebase Hosting configuration to serve the web app from the /app route, and updates the CI/CD pipeline to automatically build and deploy the web app. It also fixes an issue in the existing CI configs where an invalid flutter version was specified.

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PR created automatically by Jules for task 7018115072772569370 started by @Salint

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[github-actions]

Visit the preview URL for this PR (updated for commit b95da60):

https://financial-planner-72109--pr98-web-app-version-7018-tmt4ys1l.web.app

_{(expires Sat, 02 May 2026 04:16:01 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}

_{Sign: 95ab667b75b8af6cdab808fc3f3df54a184777c9}