[TASK] chore: rewrite gitignore and define .env.example

[MikeNovikoff]

Description

we have gitignore in /mobile and gitignore in main directory, in /mobile a lot of points, in main - only few lines. We need to move lines from /mobile to main directory.
Also we need to make .env.example template with example variables for project setup

To do

• update gitignore in /mobile and main directory
• collect requirements from teammates and struct .env.example
• optional: make requirements.txt
• updated main and dev branch and current pr not conflicting

Notes

Acceptance Criteria

• updated gitignore to prevent copied lines, scoped gitignore only for /mobile, prevent pushing openrouter keys
• .env.example created and filled correctly
• basic requirements file created - without unnecessary dependencies

[ustkost]

.env examples are already merged to the main as a part of #16