Standardize all filter and segment modals styles (#6298)

* Standardize all filter and segment modals styles

- Add a shared ModalLayout/ModalFooter to give every modal the same header, body spacing, and button alignment
- Add a shared Button component (matching the Phoenix variant) and switch all modal buttons to it for consistent sizing and theming
- Align footer buttons to the right in all modal footers
- Restructure the segment details modal: fixed "Segment details" title, with the segment name, type, and authorship shown inline in the body
- Update the delete segment modal with a clearer title and a confirmation message
- Make filter pills better visible inside modals with a gray background
- Add a Cancel button to the filter modal when no filters are applied
- Refine button theme borders so ghost, yellow, and icon variants line up at the same height as bordered buttons, and fix the visible border on disabled primary buttons

* Dark mode and responsive fixes

* Implement feedback

- Move shared button styles into `.btn-*` Tailwind component classes used by both the React Button and the Phoenix `button` component
- Tweak filter pill styling inside modals for dark mode