change placement of topbar elements tooltip to bottom-start

[bcotrim]

Related issues

• Fixes https://github.com/Automattic/dotcom-forge/issues/9896

Proposed Changes

• Added a default tooltip placement constant (DEFAULT_TOOLTIP_PLACEMENT) set to 'bottom-start'
• Standardized tooltip placement across the top bar components:
  • Applied consistent placement to the sidebar toggle tooltip
  • Applied consistent placement to the offline indicator tooltip

Testing Instructions

1. Launch the application

2. Test in LTR languages:

   • Hover over the sidebar toggle button in the top bar
     • Verify the tooltip appears from the bottom-start position
   • Toggle offline mode and hover over the offline indicator
     • Verify the tooltip appears from the bottom-start position

3. Test in RTL languages:

   • Switch the application language to an RTL language (e.g., Arabic, Hebrew)
   • Verify tooltip placement remains correct for:
     • Sidebar toggle button
     • Offline indicator
   • Ensure tooltips don't overlap with other UI elements
   • Confirm tooltips respect the RTL text direction

4. General checks:

   • Ensure tooltips remain visible and don't get cut off by screen edges
   • Verify consistent behavior across different screen sizes

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[katinthehatsite]

The changes look good to me and work as expected.

We might also consider unifying it with other files e.g. content-tab-snapshots and content-tab-sync where are also using this placement for tooltip. We can also probably move the constant to constants file.

I am approving this PR though as it fixes the original issue.