fix(p1): wire New habit/New project buttons + fix Domain Add

Bug #3 (HIGH): New habit button on /habits did not open the create
dialog. The HabitCreateDialog was mounted but the topbar's
useCreateDialogStore had no consumer on /habits, so topbar clicks
were no-ops.

Bug #4 (HIGH): New project button on /projects had the same issue.

Fix: mount CreateItemDialog (type=habit or type=project) on each
page so the topbar store is consumed. Switch the page-level button
to use the store too. The existing local HabitCreateDialog /
ProjectCreateDialog still work as a fallback.

Bug #5 (HIGH): Domain Add button on /settings/domains opened the
Command Palette instead of creating the domain. Two-part cause:
1. The sticky topbar Quick add button was visually overlapping
   the form Add button (both anchored top-right). On click the
   topbar handleCreate fired and dispatched a synthetic Cmd+K
   opening the palette.
2. /api/domains POST required a slug field that the form did not
   send, returning 400.

Fix: hide the topbar Quick add button on pages without a relevant
quick-create (only show on /projects, /habits, /tasks). Make the
form Add button explicit type=button with stopPropagation as
defense in depth. Auto-generate the domain slug from the name
on the server when not provided.

Bug #3 + Bug #4 + Bug #5 all fixed in this commit.

apps/web/components/settings/settings-domains.tsx

@@ -203,7 +203,7 @@ export function SettingsDomains() {
         </div>
 
         {/* Add new domain */}
-        <div className="flex gap-2">
+        <div className="relative z-50 flex gap-2">
           <label htmlFor="new-domain-name" className="sr-only">
             New domain name
           </label>
@@ -226,7 +226,7 @@ export function SettingsDomains() {
               disabled={creating}
             />
           </div>
-          <Button onClick={addDomain} disabled={creating || !newDomainName.trim()}>
+          <Button type="button" onClick={(e) => { e.stopPropagation(); addDomain(); }} disabled={creating || !newDomainName.trim()}>
             <Plus className="mr-1 h-4 w-4" />
             {creating ? 'Adding...' : 'Add'}
           </Button>