enhance(ui): use normal line-height for buttons and selection dropdowns (#39796)

The legacy Fomantic `line-height: 1` on buttons made ellipsized button
text clip descenders like `g` letters, like in the PR target branch
selector. Selection dropdowns were 3px taller than the inputs and
buttons next to them because their padding assumed `line-height: 1em`.

- Buttons and selection dropdowns use `--line-height-default` with
smaller vertical padding, `min-height` keeps their sizes for every font
including CJK
- Remove overrides that only compensated for the old values and dropdown
declarations that never applied
- Label-edit and push-mirror modals use regular-size action buttons like
all other modals

web_src/css/repo.css

@@ -1726,7 +1726,6 @@ tbody.commit-list {
 .branch-selector-dropdown .ui.button.branch-dropdown-button {
   margin: 0;
   max-width: 340px;
-  line-height: var(--line-height-default);
   padding: 0 0.5em 0 0.75em;
 }