appview/repo/search: improve ui/ux
- use an `<input>` instead of a `<button>`
- add support for `?q=%s` in `repo/search`
- show empty state message with query examples
- consistent `search-code` icon accross `repo/search` and `repo`
overview to distinguish from global search
resolves
[TAN-534](https://linear.app/tangled/issue/TAN-534/reposearch-nitpicks-and-reworks)
Signed-off-by: eti <eti@eti.tf>