Add sender context mode for text generation (#104)

Add a per-room/global `sender_context_mode` setting that optionally
prefixes conversation messages with sender metadata before sending them
to the model provider.

This helps models distinguish between participants in multi-user rooms.
                                                                                                                                                                                                                                                             
Three modes are supported:
- `disabled` (default, no change)
- `matrix_user_id` (prefixes with `[sender=@user:server]`)
- `matrix_user_id_and_timestamp` (adds `send_at`; Example: `[sender=@user:server sent_at=<ISO 8601>]`)
                                                                                                                                                                                                                                                             
Sender context is applied to user and assistant text messages only,
skipping system prompts and non-text content.

Mixed-sender merged turns (something we intentionally do for Anthropic)
have their `sender_id` cleared to avoid misattribution.

src/strings/help/cfg.rs

@@ -132,6 +132,18 @@ pub fn text_generation_context_management_intro() -> String {
     )
 }
 
+pub fn text_generation_sender_context_heading() -> &'static str {
+    "👤 Sender Context Mode"
+}
+
+pub fn text_generation_sender_context_intro() -> String {
+    format!(
+        "{}\n{}",
+        "Controls whether the bot attaches sender information to conversation messages before sending them to the model.",
+        "`disabled` leaves messages unchanged, `matrix_user_id` adds `[sender=@alice:example.com]`, and `matrix_user_id_and_timestamp` adds `[sender=@alice:example.com sent_at=2026-03-23T14:30:00Z]`. Enabling this sends Matrix user IDs, and optionally timestamps, to the model provider.",
+    )
+}
+
 pub fn text_generation_prompt_override_heading() -> &'static str {
     "⌨️ Prompt Override"
 }