[Improvement] Combobox: configurable popover placement

Adds a `placement` value (default `bottom-start`, unchanged behavior) so
consumers can position the popover differently, e.g. `left-start`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: pierry01

gem/lib/ruby_ui/combobox/combobox.rb

@@ -2,8 +2,9 @@
 
 module RubyUI
   class Combobox < Base
-    def initialize(term: nil, **)
+    def initialize(term: nil, placement: "bottom-start", **)
       @term = term
+      @placement = placement
       super(**)
     end
 
@@ -19,6 +20,7 @@ def default_attrs
         data: {
           controller: "ruby-ui--combobox",
           ruby_ui__combobox_term_value: @term,
+          ruby_ui__combobox_placement_value: @placement,
           action: "turbo:morph@window->ruby-ui--combobox#updateTriggerContent"
         }
       }

gem/lib/ruby_ui/combobox/combobox_controller.js

@@ -5,7 +5,8 @@ import { computePosition, autoUpdate, offset, flip } from "@floating-ui/dom";
 export default class extends Controller {
   static values = {
     term: String,
-    minPopoverWidth: { type: Number, default: 240 }
+    minPopoverWidth: { type: Number, default: 240 },
+    placement: { type: String, default: 'bottom-start' }
   }
 
   static targets = [
@@ -175,7 +176,7 @@ export default class extends Controller {
   updatePopoverPosition() {
     this.cleanup = autoUpdate(this.triggerTarget, this.popoverTarget, () => {
       computePosition(this.triggerTarget, this.popoverTarget, {
-        placement: 'bottom-start',
+        placement: this.placementValue,
         middleware: [offset(4), flip()],
       }).then(({ x, y }) => {
         Object.assign(this.popoverTarget.style, {