Changed char set (see #77)

[ggggg]

This template is a simple version of Zulip PR template: https://github.com/zulip/zulip/

Changed character set: from lowercase letters and numbers to just uppercase letters to improve readability.

Fixes: #77

Screenshots and screen captures:

Self-review checklist

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review:

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes (what you are trying to accomplish? e.g. don't let user's age less than 0).

Completed manual review and testing of the following (please attach screenshots/gifs for UI changes and commit your test cases for functionality changes):

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[shubhbapna]

LGTM