Add search functionality to Mail TUI with / keybinding

- Add reusable SearchScreen modal and ClearableSearchInput widget
- Implement filter_by_query in MessageStore for client-side filtering
- Search matches subject, sender name/email, recipient name/email
- Press / to open search, Escape to clear search filter
- Shows search query in list subtitle when filter is active

src/utils/search.py

@@ -0,0 +1,179 @@
+"""Reusable search input screen for TUI apps.
+
+A modal input dialog that can be used for search across all apps.
+"""
+
+from typing import Optional
+
+from textual.app import ComposeResult
+from textual.binding import Binding
+from textual.containers import Vertical, Horizontal
+from textual.screen import ModalScreen
+from textual.widgets import Input, Static, Label, Button
+
+
+class SearchScreen(ModalScreen[str | None]):
+    """A modal screen for search input.
+
+    Returns the search query string on submit, or None on cancel.
+    """
+
+    DEFAULT_CSS = """
+    SearchScreen {
+        align: center middle;
+    }
+    
+    SearchScreen > Vertical {
+        width: 60;
+        height: auto;
+        border: solid $primary;
+        background: $surface;
+        padding: 1 2;
+    }
+    
+    SearchScreen > Vertical > Label {
+        margin-bottom: 1;
+    }
+    
+    SearchScreen > Vertical > Input {
+        margin-bottom: 1;
+    }
+    
+    SearchScreen > Vertical > Horizontal {
+        height: auto;
+        align: right middle;
+    }
+    
+    SearchScreen > Vertical > Horizontal > Button {
+        margin-left: 1;
+    }
+    """
+
+    BINDINGS = [
+        Binding("escape", "cancel", "Cancel", show=True),
+    ]
+
+    def __init__(
+        self,
+        title: str = "Search",
+        placeholder: str = "Enter search query...",
+        initial_value: str = "",
+        name: Optional[str] = None,
+        id: Optional[str] = None,
+        classes: Optional[str] = None,
+    ) -> None:
+        super().__init__(name=name, id=id, classes=classes)
+        self._title = title
+        self._placeholder = placeholder
+        self._initial_value = initial_value
+
+    def compose(self) -> ComposeResult:
+        with Vertical():
+            yield Label(self._title)
+            yield Input(
+                placeholder=self._placeholder,
+                value=self._initial_value,
+                id="search-input",
+            )
+            with Horizontal():
+                yield Button("Search", variant="primary", id="search-btn")
+                yield Button("Cancel", variant="default", id="cancel-btn")
+
+    def on_mount(self) -> None:
+        """Focus the input on mount."""
+        self.query_one("#search-input", Input).focus()
+
+    def on_input_submitted(self, event: Input.Submitted) -> None:
+        """Handle Enter key in input."""
+        self.dismiss(event.value)
+
+    def on_button_pressed(self, event: Button.Pressed) -> None:
+        """Handle button presses."""
+        if event.button.id == "search-btn":
+            query = self.query_one("#search-input", Input).value
+            self.dismiss(query)
+        elif event.button.id == "cancel-btn":
+            self.dismiss(None)
+
+    def action_cancel(self) -> None:
+        """Cancel the search."""
+        self.dismiss(None)
+
+
+class ClearableSearchInput(Static):
+    """A search input widget with clear button for use in sidebars/headers.
+
+    Emits SearchInput.Submitted message when user submits a query.
+    Emits SearchInput.Cleared message when user clears the search.
+    """
+
+    DEFAULT_CSS = """
+    ClearableSearchInput {
+        height: 3;
+        padding: 0 1;
+    }
+    
+    ClearableSearchInput > Horizontal {
+        height: auto;
+    }
+    
+    ClearableSearchInput > Horizontal > Input {
+        width: 1fr;
+    }
+    
+    ClearableSearchInput > Horizontal > Button {
+        width: 3;
+        min-width: 3;
+    }
+    """
+
+    from textual.message import Message
+
+    class Submitted(Message):
+        """Search query was submitted."""
+
+        def __init__(self, query: str) -> None:
+            super().__init__()
+            self.query = query
+
+    class Cleared(Message):
+        """Search was cleared."""
+
+        pass
+
+    def __init__(
+        self,
+        placeholder: str = "Search...",
+        name: Optional[str] = None,
+        id: Optional[str] = None,
+        classes: Optional[str] = None,
+    ) -> None:
+        super().__init__(name=name, id=id, classes=classes)
+        self._placeholder = placeholder
+
+    def compose(self) -> ComposeResult:
+        with Horizontal():
+            yield Input(placeholder=self._placeholder, id="search-input")
+            yield Button("X", id="clear-btn", variant="error")
+
+    def on_input_submitted(self, event: Input.Submitted) -> None:
+        """Handle search submission."""
+        self.post_message(self.Submitted(event.value))
+
+    def on_button_pressed(self, event: Button.Pressed) -> None:
+        """Handle clear button."""
+        if event.button.id == "clear-btn":
+            input_widget = self.query_one("#search-input", Input)
+            input_widget.value = ""
+            input_widget.focus()
+            self.post_message(self.Cleared())
+
+    @property
+    def value(self) -> str:
+        """Get the current search value."""
+        return self.query_one("#search-input", Input).value
+
+    @value.setter
+    def value(self, new_value: str) -> None:
+        """Set the search value."""
+        self.query_one("#search-input", Input).value = new_value