refactor: complete TextFieldState migration — remove all dual TextFieldValue properties

Remove the old `var message by mutableStateOf(TextFieldValue(""))` properties
from all ViewModels, making `TextFieldState` the single source of truth.

Key changes:
- Add TextFieldState extension functions (updateText, insertUrlAtCursor,
  replaceCurrentWord, currentWord) to TextFieldValueExtensions.kt
- Update PreviewState to accept String instead of TextFieldValue
- Add TextFieldState overload to UserSuggestionState.replaceCurrentWord
- Remove forwardZapToEditting and updateZapForwardTo from IZapField interface
- Migrate all 8 ViewModels: EditPostViewModel, ShortNotePostViewModel,
  ChatNewMessageViewModel, NewPublicMessageViewModel, CommentPostViewModel,
  LongFormPostViewModel, NewProductViewModel, ChannelNewMessageViewModel
- Update all Screen composables that referenced removed properties

https://claude.ai/code/session_01VminH5nD1jbzncbgTaqXEK

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/note/creators/previews/PreviewState.kt

@@ -21,7 +21,6 @@
 package com.vitorpamplona.amethyst.ui.note.creators.previews
 
 import androidx.compose.runtime.Stable
-import androidx.compose.ui.text.input.TextFieldValue
 import com.vitorpamplona.amethyst.service.CachedUrlParser
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.FlowPreview
@@ -33,22 +32,22 @@ import kotlinx.coroutines.flow.map
 
 @Stable
 class PreviewState {
-    var source = MutableStateFlow(TextFieldValue(""))
+    var source = MutableStateFlow("")
 
     @OptIn(FlowPreview::class)
     val results =
         source
             .debounce(500)
             .map {
-                CachedUrlParser.parseValidUrls(it.text)
+                CachedUrlParser.parseValidUrls(it)
             }.distinctUntilChanged()
             .flowOn(Dispatchers.IO)
 
     fun reset() {
-        source.tryEmit(TextFieldValue(""))
+        source.tryEmit("")
     }
 
-    fun update(text: TextFieldValue) {
+    fun update(text: String) {
         source.tryEmit(text)
     }
 }

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/note/creators/zapsplits/IZapField.kt

@@ -23,13 +23,11 @@ package com.vitorpamplona.amethyst.ui.note.creators.zapsplits
 import androidx.compose.foundation.text.input.TextFieldState
 import androidx.compose.runtime.MutableState
 import androidx.compose.runtime.Stable
-import androidx.compose.ui.text.input.TextFieldValue
 import com.vitorpamplona.amethyst.model.User
 
 @Stable
 interface IZapField {
     val forwardZapTo: MutableState<SplitBuilder<User>>
-    val forwardZapToEditting: MutableState<TextFieldValue>
     val forwardZapToEdittingState: TextFieldState
 
     fun updateZapFromText()
@@ -38,6 +36,4 @@ interface IZapField {
         index: Int,
         sliderValue: Float,
     )
-
-    fun updateZapForwardTo(newZapForwardTo: TextFieldValue)
 }