feat(config-settings): inline in-field value editor (no fish read prompt)

Replace the fish `read` prompt (which showed an unstyled `read>` and left the
prompt line behind on redraw) with an in-place editor built on the raw key
reader. The value is edited directly in its UI field with a block caret; the
panel redraws each keystroke and cleans up on exit. Pre-fills the current value;
Backspace deletes, Enter saves (empty reverts to default), Esc cancels.

- __config_settings_read_key: decode Backspace (bytes 8/127)
- __config_settings_draw_value: edit-mode field with caret + edit hint line
- config-settings: inline edit loop replaces the read-based prompt block

functions/__config_settings_read_key.fish

@@ -24,7 +24,7 @@
 # RETURNS
 #   0  A key was read; one of these tokens is printed to stdout:
 #        up down left right   arrow keys
-#        space tab backtab enter escape
+#        space tab backtab enter escape backspace
 #        quit                 Ctrl-C (byte 3) in raw mode
 #        <char>               any other single printable character
 #        ""                   nothing decodable was read
@@ -74,6 +74,8 @@ function __config_settings_read_key
             echo space
         case 10 13
             echo enter
+        case 8 127
+            echo backspace
         case 3
             echo quit
         case ''