Move Issue and PR buttons in navbar from left to right

[lutinglt]

1. Move the Issue and PR buttons in the navbar, which are highly user-relevant, to the right side, and place the less user-relevant items on the left.
2. Position the Create menu as the first item on the right for better aesthetics.
3. Replace the notification icon from octicon-bell to octicon-inbox to align with the icon used when there are no notifications.
4. Set a fixed height for the notification icon to keep it vertically centered and aligned with other icons.

This style aligns more closely with GitHub's design.

before:

after:

mobile (320x738):

no notifications & icon height
before:

after:

[silverwind]

Seems a good change, my only worry is the the left side is a bit empty after this. Maybe later we could move those somewhere else and use the empty room as the page title.

[lutinglt]

Seems a good change, my only worry is the the left side is a bit empty after this. Maybe later we could move those somewhere else and use the empty room as the page title.

Custom links can be added on the left side, it may not be easy to move.

[silverwind]

I guess custom links could then be moved to the right side if we make use of the left side for title.

[lunny]

But why are milestones placed on the left? For future features like Packages or a search box, should those also be moved to the right?

[silverwind]

Hmm, the left side of the header would then be empty if we move those as well.

[lutinglt]

Place items strongly related to individual users on the right, and those weakly related on the left.

User avatars should be on the right, and action buttons on the right should be user-related. Issues, PRs, and notifications follow the same logic—users typically want to view content related to themselves.

However, milestones are clearly an exception. Although milestones are somewhat related to users, they are more relevant to the repository or organization. Those who need to view milestones are specialized users (e.g., product managers), not every user. Therefore, milestones should be placed on the left.

This way, when users view content related to themselves, they don't need to check the left side but can focus solely on the right.

[lutinglt]

The icons on the right side of the top navigation bar should contain high-frequency actions that every user will perform, while low-frequency operations can be placed in the user dropdown menu. The same logic can be applied to the left side.

In the future, if the left side is intended to be used as a title section, it might be better to collapse all left-side actions into a left-hand menu, similar to how GitHub does it.

[lutinglt]

Will this PR be merged? If not, I will close it.

[silverwind]

I can accept it as-is. Not sure about the others.