feat(ui): add SelectMenu component and replace native selects

Introduce a reusable SelectMenu component and replace native select
controls across the UI. This centralizes select behavior and styling,
enabling grouped options, placeholders, and a consistent popup
interaction. Add comprehensive CSS and light-theme tweaks, and update
rebase action styling to integrate the new control.

- Add a unified SelectMenu component and wire change handlers.
- Implement .select-menu styles, popup behavior, and theme overrides.
- Replace ad-hoc native selects in dialogs and rebase UI for consistency.

src/lib/components/WorktreeDialog.svelte

@@ -21,6 +21,7 @@
     X,
   } from "@lucide/svelte";
   import type { GitBranch as GitBranchInfo, GitWorktree } from "../types";
+  import SelectMenu from "./SelectMenu.svelte";
 
   type CreateMode = "existing" | "new" | "detached";
 
@@ -260,12 +261,17 @@
             {#if createMode === "existing"}
               <label>
                 <span>Branch</span>
-                <select bind:value={selectedBranch} disabled={isBusy}>
-                  <option value="" disabled>Select a local branch</option>
-                  {#each localBranches as branch (branch.name)}
-                    <option value={branch.name} disabled={!branchAvailable(branch.name)}>{branch.name}{!branchAvailable(branch.name) ? " (already checked out)" : ""}</option>
-                  {/each}
-                </select>
+                <SelectMenu
+                  value={selectedBranch}
+                  placeholder="Select a local branch"
+                  options={localBranches.map((branch) => ({
+                    value: branch.name,
+                    label: `${branch.name}${!branchAvailable(branch.name) ? " (already checked out)" : ""}`,
+                    disabled: !branchAvailable(branch.name),
+                  }))}
+                  disabled={isBusy}
+                  onChange={(value) => { selectedBranch = value; }}
+                />
               </label>
             {:else if createMode === "new"}
               <label>