feat: give the app a navigation component, and stop the app bar duplicating it

Phase 6, step 3. The app had no navigation component of any kind: 43 screens
reached by pushing a route, and one home screen whose top app bar carried the
only two peer surfaces -- a profile avatar in the leading slot, a search icon in
the trailing one.

**This is an information-architecture change and was taken as one.** With a
single top-level destination, a navigation bar would have held one item and been
strictly worse than the app bar it replaced -- M3's caution is to swap only
functionally equivalent components. Promoting search and profile to peer
destinations is what makes a navigation component mean anything here, and it was
put to the product owner rather than inferred. Answered: promote them.

The consequence is in `HomeScreen`: the app bar now carries a title and nothing
else. Two routes to one destination is the thing the caution is about, and the
navigation component is now the one route, at every breakpoint.

**Which component, at which breakpoint**, straight from the layout foundation:

  | compact            | navigation bar |
  | medium, expanded   | collapsed rail |
  | large, extra-large | expanded rail  |

`NavigationSuiteScaffoldDefaults.navigationSuiteType` is not used, and the
difference is the last row -- it stops at the collapsed rail, because it
classifies with the three-value window size class rather than the five
breakpoints the May 2026 revision published. Deriving from `Breakpoint` reaches
the row the library's default cannot, and keeps one source of truth for window
width in the app.

`NavigationSuiteType.None` on everything else. A navigation bar belongs on the
destinations it switches between; on a chat room, a signing screen or an
onboarding step -- pushed to and left by coming back -- it is a permanent
invitation to lose your place.

**Two things the wiring needed.**

`ActiveProfileRoute` is addressed by metadata event id, not by public key, and
only the home screen ever had one. The nav host now observes it for as long as a
key is signed in, and the profile item is *disabled* until it arrives rather than
absent -- an item that appears late moves the two beside it, and a bar whose
items move under a thumb is worse than one briefly unavailable.

The item click pops to `HomeRoute`, not to the graph's start destination. The
android docs give the second shape and it would be wrong here: this graph starts
at `LoadingRoute`, and onboarding clears the stack with `popUpTo(0)` on its way
to home, so by the time these items exist the start destination is not on the
stack at all -- popping to it would leave the loading screen underneath as the
thing back returns to.

**Tests, and one that could not be written.** The breakpoint-to-component table
is a pure function so all five rows are asserted; the two rail rows differ only
in whether labels are drawn, and nobody opens a 1200dp window on purpose. Four
more compose the component around a real nav graph, because `TopLevelDestination.of`
matches by `hasRoute` -- reflection over the serialized route -- and a renamed
route would fail by never showing the component at all.

Navigation in those is driven through the controller rather than by tapping an
item. That is a harness limitation, established rather than assumed: a click
handler that navigates trips navigation-compose's own main-thread assertion under
`runDesktopComposeUiTest`, reproducible in twenty lines containing no app code --
a `NavHost`, two routes and a `TextButton`. What an item's `onClick` builds is
asserted where it is a pure function instead.

Also `material3-adaptive-navigation-suite`, versioned with material3 rather than
with the adaptive library: it is published by the material3 group, and its
1.10.0-alpha05 is what names adaptive 1.2.0 in the first place.

Most of the `MantraNavHost` diff is indentation -- the `NavHost` call gained an
enclosing composable. `git diff -w` shows the 38 lines that are not.

626 jvm tests green; android compiles.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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

@@ -158,6 +158,7 @@
     <string name="post">Post</string>
     <string name="post_functionality_coming_soon">post functionality coming soon.</string>
     <string name="private_to_you">Private to you</string>
+    <string name="messages">Messages</string>
     <string name="profile">Profile</string>
     <string name="profile_is_ready">Profile is ready</string>
     <string name="profile_keys">Profile keys</string>