explore: the first rows on a phone's first screen; Make it yours only where it can be made #930

Closes #922

On a phone, /explore shows its first rows on the first screen. A row offers Make it yours only where the viewer may make it theirs.

• Phone shape (under 600 px):

- the lead is shorter; - the kind and sort chips share one row that scrolls sideways; - the Make it yours button sits under a row's numbers instead of squeezing them into three lines.

• Desktop is unchanged apart from the shorter lead.
• Who sees the button: the row now carries copyable (on the page and in the API), worked out by the rules Make it yours itself checks:

- the viewer is a signed-in person who is not suspended and not the owner; - the app is public, or a private app the viewer enters as its own. - Before, a reader saw the button on a public repository's private app and was then refused.

• Test: test/explore.test.ts, #922: the button is absent for a reader on a gated app and for the owner on their own app, and present on a public app.
• Looks: at 390 px (iPhone 15) for anonymous and signed-in visitors, and at 1280 px; no horizontal overflow at either width.

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