refactor: promote BrowserPanelView computed vars to standalone subviews (#68)

Nuclear-review finding WP6 (lowest severity): BrowserPanelView was a single
~1900-line View struct decomposed only via private computed vars, making it
hard to scan and hard for the type checker to isolate errors.

Promotes the 4 largest cohesive computed-var groups into standalone private
struct View types in the same file, taking explicit `let`/`@Binding` params
instead of implicitly reading the parent's state:

- BrowserNavigationButtonsView (back/forward/reload/stop + download
  indicator) - 1 param (panel)
- BrowserProfileMenuView (profile toolbar button + popover) - 6 params
- BrowserThemeModeMenuView (theme toolbar button + popover) - 4 params
- BrowserImportHintContentView (blank-tab card/inline-strip overlays +
  toolbar-chip popover, which all share the same hint body) - 4 params

Body content moved verbatim; behavior, timing, and state ownership are
unchanged. Groups that would need more than ~8 parameters to extract
cleanly (omnibarField, webView, and the top-level addressBar composition)
were left in place per the conservative scope of this pass.

BrowserPanelView's own body shrinks from ~1913 to ~1604 lines.

Author: arzafran