fix: allow editor focus when project search drawer is open

[layer78]

Focus gets stuck on the project search input when using "Find in project".
After clicking a search result or clicking the editor text area, typing
still goes to the search box instead of the editor.

The root cause is update_focus() unconditionally focusing the project
search input whenever the context drawer is open on the ProjectSearch page.
Unlike the Find bar which has a has_focus flag to allow focus to return
to the editor, the project search had no equivalent mechanism.

This adds a project_search_has_focus flag mirroring the existing Find bar
pattern:

• Set true when opening the search drawer, typing in the search input,
  or receiving search results
• Set false when clicking a search result or clicking the editor
• update_focus() only focuses the search input when the flag is true

Steps to reproduce (before fix):

1. Open a project
2. Edit -> Find in project...
3. Search for something
4. Click a result
5. Click in the editor text area
6. Start typing — input goes to the search box instead of the editor

• I have disclosed use of any AI generated code in my commit messages.
• I understand these changes in full and will be able to respond to review comments.
• My change is accurately described in the commit message.
• My contribution is tested and working as described.
• I have read the Developer Certificate of Origin and certify my contribution under its conditions.

[jacobgkau]

Confirmed this fixes focus with the project search drawer open. Thank you for the fix!

Regression testing passed:

Basic operations

• Type three lines of text (no trailing newline).
• Copy the last two lines.
• Paste at the end of the third line (file should now have four lines).
• Ctrl-Z to undo the paste.
• Press Enter to add a trailing newline.
• Paste again (file should now have five lines).
• Ctrl-Z to undo the paste.
• Ctrl-Shift-Z to redo the paste.
• Save the file.
• Ctrl-F and search for something that has a match.
• Press Esc twice to exit the Find dialog.
• Press Ctrl-X to cut the selected search result.
• Paste the cut text on a new line (file should now have six lines).
• Re-save the file.
• Narrow the window until the lines start wrapping (make a line longer if necessary to observe line wrapping).
• Turn word wrapping off.
• Scroll right to the end of the document, then left to the beginning again.
  • Active line highlight stops after visible part scrolls; not a regression.
• Click and drag to select some text past the horizontal edge of the window.
• Close the file, open COSMIC Edit again, and open the file via the recents list.
  • Doesn't show up; not a regression: #460
• Close the file again, open COSMIC Edit again, and open the file via the Open dialog.
• Turn word wrapping back on.

Settings

• Open View -> Settings.
• All Appearance settings work.
• Vim bindings work.

Projects & Git Management

• Clone the cosmic-edit Git repo and open its directory as a project.
• Edit -> Find in project... works.
• Make a change in a file.
• File -> Git management shows the change and staging it works.
• Make another change while Git management's open; it updates to show the new change.
• Un-staging a change works.

Additional Usage

• Open a large file (e.g. sudo dmesg > dmesg.txt), click and drag to select all of its text, cut, and paste.