Svelte: Milestone 3 Umbrella form small UI/UX improvements

[vovakulikov]

For milestone 3 (June release) we want to address the following list of
UI/UX problems

• Add a hover state for the search result block's header to solve the problem of aiming the action buttons at the end of the row on big screens (Slack message with feedback)
• Add custom platform identical scrollbars for scrollable blocks
• Modify panel separators/dividers to act like Slack (Svelte: First run on file tree visual update #62585 (comment))
• Show file icon in file header in diff mode
• Set minimum header height so that file header has the same size in diff view and file view (right now the header appears to "shift").