Svelte: add repo header dropdown menu

[camdencheek]

Stacked on #63253

Completes SRCH-22 (at least most of it)

Unimplemented from the designs:

• Mobile layout. I'm not exactly sure what the header in general should look like on mobile, but it should be pretty straightforward to move the tab items to the menu when we have that.
• Contributors. We do not have that data available cheaply.

This adds the dropdown menu for the repo name button.

And the overflow menu (contains some icon and padding changes):

Test plan

Manual testing of buttons, links, and keyboard shortcuts. Added playwright tests for the meaningful interactions with this menu.

[camdencheek]

Increased these to match the designs.

[camdencheek]

@taiyab small design feedback: with the menu open, that part of the page looks pretty busy and the dropdown menu does not stand out well against the file tree. This is especially true in dark mode (visible in the screenshot). Maybe a more aggressive shadow? Or a different placement of the menu?

[fkling]

We could consider to have RepoMenu take in a Repository GraphQL object and have it define it's own dependencies in a fragment. That would mean the component needs to compute values such as the repo URL and the display name itself but it would be more encapsulated.
No strong feelings about either solution though.

[taiyab]

@camdencheek I've added an updated shadow style to the menus, both for light and dark, which should help differentiate it from the background.

https://www.figma.com/design/EPgl0qVbqGSumvf52brhda/Code-Search?m=auto&node-id=4119-422020&t=MJYz4JIVjK5BosZw-1

[camdencheek]

I've added an updated shadow style to the menus,

Much better, thanks!