fix(ui): a back button on every pushed screen, and one spelling of it

Eighteen screens drew a back arrow in their app bar's leading slot and twenty-two
pushed screens did not. On Android the system back stood in for it and on iOS the
edge swipe, but the desktop target has neither, and a screen reached by
`navigate(...)` with no way to pop it is a dead end there: seven group forms, six
chat screens, five DKG screens, and four screens with no app bar at all.

**One widget, `NavigateBackButton`, rather than a nineteenth inline copy.** The
eighteen that had the button spelled it four ways -- `Icons.Filled.ArrowBack`, the
auto-mirrored one, `ArrowBackIosNew`, and a `"Back"` literal against a
`stringResource`. The widget decides twice: the icon is the auto-mirrored one,
because "back" points at the leading edge and the leading edge is on the right in an
RTL locale, which is what `Icons.Filled.ArrowBack` is deprecated for; and the
description is the catalogue's, because it is text. All forty-one sites use it now,
the eighteen converted mechanically with the imports they no longer need dropped.

**Each screen takes `onNavigateBack` and the host passes `popBackStack()`.** Hoisted
rather than read from a controller in the screen, so every one stays previewable
and testable without a nav host, which is how the twenty-two were fixed without a
nav host in a single test.

**Four screens had no bar to put it in, and got one.** `ChatRoomCreationScreen` is
"New chat", after the button that opens it. `CreateProfileScreen` is "Create
profile", and the two body headlines that repeated the name are gone, which is the
shape `SignInScreen` beside it on the landing page already has. `WriteNewNoteScreen`
is "New note" whether it is a reply, a quote or neither, one new string. The
"coming soon" placeholder's bar is titled with the name of what was tapped.
`AddMemberToChatRoomConfirmationScreen`'s bar had been commented out; it is back,
titled "Invite new member" since the body already names who and which room.
`NostrEventDetailScreen`'s repost and unknown-kind branches were the last two
placeholders without a bar.

**Two screens are reached two ways, and only the caller knows which.** Their
callback is nullable, and null draws no button. `ChatRoomMessagingScreen` is a
destination on a phone and the home screen's detail pane in an expanded window;
in the pane the room list is beside the transcript and a button that popped would
pop the home screen, so the pane passes null. `ImplementationPendingScreen` is
pushed from "learn more" and "edit profile" and is also where the navigation
observer lands with `popUpTo(0)` on an error; the host reads
`previousBackStackEntry`, remembered at first composition because the departing
screen reads it again after the stack has moved on. The DKG approval screens reuse
their existing `onDone`, which "Not now" already called -- the bar makes the same
leave reachable from the loading and error states, which had no other way off.

**Left alone on purpose.** The three top-level destinations have the navigation
bar, which phase 6 put there instead of app-bar icons. The onboarding and loading
screens cleared the stack to get where they are and have nothing under them.
`SearchScreen` and `SearchResultScreen` keep their `ArrowBackIosNew`: it is a search
bar's collapse control in a `leadingIcon` slot, not a navigation icon.

`NavigateBackButtonJvmTest` covers the two conditional screens by the description a
screen reader would announce: present and popping when pushed, absent in the pane
and at the root. The rule is recorded in CLAUDE.md beside the others a new screen has
to follow.

Verified with :composeApp:compileDebugKotlinAndroid, :composeApp:jvmTest (938
tests, 4 new) and docs/scripts/m3-audit.sh --check, all budgets met.

Replayed onto Mantra by docs/curated-to-mantra.md: CuratedSuggestionListScreen.kt: taken as the original merge fedbe724 left it, this being the branch join; AcceptCuratedSuggestionScreen.kt: brought to the state the original merge fedbe724 left it in, an edit that merge made outside its conflicts; BroadcastGroupSignedEventScreen.kt: brought to the state the original merge fedbe724 left it in, an edit that merge made outside its conflicts.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Pulled-From: curated/curated@0634487e12

composeApp/src/commonMain/composeResources/values/strings.xml

@@ -679,4 +679,6 @@
     <string name="you_have_not_backed_up_your_nostr_secret_key">You have not backed up your nostr secret key</string>
     <string name="these_are_your_keys_keep_them_safe_no_wallet">These are your keys. Keep them safe so they can keep unlocking this profile, even when you lose or change your phone.</string>
     <string name="you_said_you_stored_it">You said you stored it</string>
+    <!-- The bar on the note composer, which had no bar before it had a back button. -->
+    <string name="new_note">New note</string>
 </resources>