refactor(settings): stop duplicating the import hint inside Settings

[arzafran]

What this does

The Browser settings section rendered its own copy of the blank-tab import hint, reusing the hint's real strings. That included the hint's footnote, "You can always find this in Settings > Browser", shown to someone already standing in Settings.

The Choose and Refresh buttons immediately below it already offer the action the mock was advertising, so the mock was only restating them with extra chrome. The real hint card in BrowserToolbarViews is untouched, and both strings are still used there.

Summary

• Removed the mock card and its rounded-rect background/border from browserSection
• Removed browserImportSubtitle, whose only caller was that mock
• Kept browser.import.hint.title and browser.import.hint.settingsFootnote, still used by the real card

Why this is smaller than the other settings cuts

Browser turned out to have far less removable surface than Sidebar Appearance or Workspace Colors, which were pure presentation. Most of this section is functional or security-relevant, so it stays:

• The HTTP allowlist governs which non-HTTPS hosts open without a warning prompt.
• Show Search Suggestions decides whether keystrokes reach the search engine.
• The host whitelist and external-URL patterns are real link-routing controls.

One thing looked cuttable and was not. Show import hint on blank browser tabs is the only path that re-enables the hint: BrowserPanelView sets both showOnBlankTabs = false and dismissed = true when you press "Hide Hint" in the browser toolbar, and the Settings binding is what clears dismissed again. Removing the toggle would have stranded anyone who ever hid the hint, with no way back short of Reset All Settings.

Test plan

• xcodebuild -scheme programa builds after rebase
• Both hint strings confirmed still referenced by Sources/Panels/BrowserToolbarViews.swift
• Open Settings, Browser tab, confirm the Import Browser Data block still shows its heading, Choose/Refresh, and the hint toggle

Note

Stacked on #232 and based on that branch, so this diff shows only the one commit. It will retarget to main automatically when #232 merges.