feat(fzf): add preview, dirs, and image support to @@ inline picker

The @@ picker only listed files with no preview. It now lists both
files and directories via fd (matching _fzf_search_directory), and
shows a bat-highlighted or image-rendered preview through
_fzf_preview_file. Image previews use a kitty-graphics-protocol,
chafa, viu, timg fallback chain via the new _fzf_preview_image
helper, benefiting the Ctrl+F directory search and git-status
pickers as well since they share the same preview helper.

functions/_fzf_preview_image.fish

@@ -0,0 +1,39 @@
+# helper function for _fzf_preview_file
+function _fzf_preview_image --description "Render an image preview using the best available tool for the current terminal."
+    set -f file_path $argv
+
+    set -l cols (set --query FZF_PREVIEW_COLUMNS; and echo $FZF_PREVIEW_COLUMNS; or echo 80)
+    set -l lines (set --query FZF_PREVIEW_LINES; and echo $FZF_PREVIEW_LINES; or echo 24)
+    set -l left (set --query FZF_PREVIEW_LEFT; and echo $FZF_PREVIEW_LEFT; or echo 0)
+    set -l top (set --query FZF_PREVIEW_TOP; and echo $FZF_PREVIEW_TOP; or echo 0)
+
+    # Terminals that implement the kitty graphics protocol (kitty itself,
+    # WezTerm, and Ghostty) can render via `kitten icat` even when it's not
+    # the active terminal, since `kitten` is just an escape-sequence emitter.
+    set -l kitty_capable 0
+    if set --query KITTY_WINDOW_ID
+        or test "$TERM" = xterm-kitty
+        or contains -- "$TERM_PROGRAM" WezTerm ghostty
+        or set --query GHOSTTY_RESOURCES_DIR
+        set kitty_capable 1
+    end
+
+    if test $kitty_capable -eq 1; and command -q kitten
+        # --place is measured from the top-left of the whole terminal, not the
+        # preview pane, so FZF_PREVIEW_LEFT/TOP (added to fzf specifically for
+        # this integration) are required to land the image in the right spot.
+        command kitten icat --clear --transfer-mode=memory --unicode-placeholder \
+            --stdin=no --place="$cols"x"$lines"@"$left"x"$top" -- "$file_path" 2>/dev/null
+    else if command -q chafa
+        command chafa --size="$cols"x"$lines" -- "$file_path"
+    else if command -q viu
+        command viu --width $cols -- "$file_path"
+    else if command -q timg
+        command timg -g "$cols"x"$lines" -- "$file_path"
+    else
+        set_color yellow
+        echo "No image preview tool available (install kitty, chafa, viu, or timg)."
+        set_color normal
+        command file --brief -- "$file_path"
+    end
+end