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/modules/input.css

@@ -91,8 +91,6 @@
   display: flex;
   align-items: center;
   flex: 0 0 auto;
-  padding-top: 0.78571429em;
-  padding-bottom: 0.78571429em;
   margin: 0;
 }
 
@@ -132,8 +130,6 @@
 
 .ui.action.input .ui.ui.button {
   border-color: var(--color-input-border);
-  padding-top: 0; /* the ".action.input" is "flex + stretch", so let the buttons layout themselves */
-  padding-bottom: 0;
 }
 
 /* currently used for search bar dropdowns in repo search and explore code */