Add a keybinding for next/previous match in thread find

[FreeWall]

Problem

Find in thread (mod+f, #10439) can only step between matches with Enter / Shift+Enter while the find field has focus, or by clicking the arrow buttons. Once focus leaves the field (after clicking into the timeline, pressing an arrow button, or reading a match), there is no keyboard way to jump to the next match without clicking back into the field. Next/previous are also not keybinding commands, so they cannot be rebound in Settings → Keybindings.

Browsers and editors use Cmd+G / Cmd+Shift+G (and F3 / Shift+F3) for this, and users reach for those keys out of habit.

Proposal

Add two keybinding commands, chat.findNext and chat.findPrevious, rebindable in Settings like any other command.

• Defaults: mod+g / mod+shift+g, with when: "threadFindOpen && !terminalFocus && !previewFocus".
• They only act while the find bar is open. With find closed the keys keep their current meaning: mod+shift+g still opens the branch picker (composer.branch), and mod+g goes to the browser.
• threadFindOpen is a new when-clause variable, following the existing modelPickerOpen pattern (modelPicker.nextProvider reuses mod+shift+arrowdown only while the picker is open).
• Terminals and previews are excluded, like chat.find, so Ctrl+G in a terminal is not captured.
• Enter / Shift+Enter inside the find field keep working as they do today.

Scope is web/desktop only (keyboard shortcuts); mobile is unaffected. No server or contract changes beyond the two new command names.

Open questions

• Is mod+g / mod+shift+g an acceptable default, given mod+shift+g is shadowed by find while the bar is open? Alternatives: F3 / Shift+F3, or no defaults at all.

I have a working implementation with a focused test and would be happy to open a PR if the direction and defaults are approved.