fix(ui): make the last five FABs circular

Audited all 76 FloatingActionButton call sites: 65 already passed
shape = CircleShape, five did not and inherited Material3's defaults --
RoundedCornerShape(16.dp) for a regular FAB, 12.dp for a small one. Both read
as rounded squares, which this app uses nowhere.

  MarkdownToolbar x2   SmallFloatingActionButton, the long-form editor's whole
                       toolbar row, which is where they are most visible
  BuzzCanvasScreen x2  plain FABs, no shape
  home/NewChessGameButton  plain FAB, no shape

The convention was only ever held together by copy-paste -- nothing enforced
it, so a FAB written from the Material3 API rather than from a neighbouring
file came out square. That is exactly how the geocaching one went wrong too.

home/NewChessGameButton appears to be an unreferenced duplicate: ChessLobbyScreen
resolves to the same-package chess/ copy, which was already circular. Its shape
is fixed here rather than deleting the file, which is a maintainer's call.

The ten ExtendedFloatingActionButtons are deliberately untouched -- those are
correctly pill-shaped, and a first pass nearly flagged them because the naive
grep matches FloatingActionButton as a substring.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WG7T9hAHDMKPaHsWvqySEr

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/buzz/BuzzCanvasScreen.kt

@@ -26,6 +26,7 @@ import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.shape.CircleShape
 import androidx.compose.foundation.text.selection.SelectionContainer
 import androidx.compose.foundation.verticalScroll
 import androidx.compose.material3.CircularProgressIndicator
@@ -158,7 +159,7 @@ fun BuzzCanvasScreen(
         },
         floatingActionButton = {
             if (canEdit) {
-                FloatingActionButton(onClick = { editing = true }) {
+                FloatingActionButton(onClick = { editing = true }, shape = CircleShape) {
                     Icon(symbol = MaterialSymbols.Edit, contentDescription = stringRes(Res.string.buzz_canvas_edit))
                 }
             }
@@ -236,6 +237,7 @@ private fun CanvasEditor(
         },
         floatingActionButton = {
             FloatingActionButton(
+                shape = CircleShape,
                 onClick = {
                     val relay =
                         RelayUrlNormalizer.normalizeOrNull(relayUrl) ?: run {

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/home/NewChessGameButton.kt

@@ -21,7 +21,9 @@
 package com.vitorpamplona.amethyst.ui.screen.loggedIn.home
 
 import androidx.activity.compose.LocalActivity
+import androidx.compose.foundation.shape.CircleShape
 import androidx.compose.material3.FloatingActionButton
+import androidx.compose.material3.MaterialTheme
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
@@ -38,6 +40,7 @@ import com.vitorpamplona.amethyst.ui.screen.loggedIn.AccountViewModel
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.chess.ChessViewModelFactory
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.chess.ChessViewModelNew
 import com.vitorpamplona.amethyst.ui.stringRes
+import com.vitorpamplona.amethyst.ui.theme.Size55Modifier
 
 /**
  * Floating action button for creating new chess game challenges
@@ -58,6 +61,9 @@ fun NewChessGameButton(
 
     FloatingActionButton(
         onClick = { showDialog = true },
+        modifier = Size55Modifier,
+        shape = CircleShape,
+        containerColor = MaterialTheme.colorScheme.primary,
     ) {
         Icon(
             symbol = MaterialSymbols.Add,

commonsUI/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/nip23LongContent/ui/editor/MarkdownToolbar.kt

@@ -25,6 +25,7 @@ import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.height
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.shape.CircleShape
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.SmallFloatingActionButton
 import androidx.compose.material3.Text
@@ -123,6 +124,7 @@ private fun ToolbarIconButton(
         onClick = onClick,
         containerColor = containerColor,
         contentColor = contentColor,
+        shape = CircleShape,
         modifier =
             Modifier
                 .size(32.dp)
@@ -159,6 +161,7 @@ private fun ToolbarButton(
         onClick = onClick,
         containerColor = containerColor,
         contentColor = contentColor,
+        shape = CircleShape,
         modifier =
             Modifier
                 .size(32.dp)