feat: crush-inspired TUI improvements, activity bar polish, startup fresh session

TUI improvements:
- semantic icon constants (✓ ◆ ▣ ✕ ⌘ ◌ ◔ ◉)
- pill-style metadata bar with model/provider/session/context
- fuzzy command palette with deterministic subsequence scoring
- input history navigation (ctrl+p / ctrl+n)
- structured tool log cards (▣ tool_name + args preview)
- no-background activity indicator and meta bar for cleaner rendering

Startup behavior:
- default startup creates a fresh session
- explicit --resume <id> restores saved context
- added main_test.go for initializeSession coverage

internal/tui/theme.go

@@ -15,6 +15,17 @@ const (
 	ThemeDark  ThemeMode = "dark"
 )
 
+const (
+	iconReady   = "✓"
+	iconWorking = "◆"
+	iconTool    = "▣"
+	iconError   = "✕"
+	iconCommand = "⌘"
+	iconSession = "◌"
+	iconContext = "◔"
+	iconModel   = "◉"
+)
+
 func ParseThemeMode(value string) (ThemeMode, error) {
 	switch ThemeMode(strings.ToLower(strings.TrimSpace(value))) {
 	case "", ThemeLight:
@@ -48,11 +59,23 @@ type styles struct {
 	App           lipgloss.Style
 	Muted         lipgloss.Style
 	Activity      lipgloss.Style
+	Header        lipgloss.Style
 	ModelMeta     lipgloss.Style
 	Footer        lipgloss.Style
 	Viewport      lipgloss.Style
 	InputBox      lipgloss.Style
 	CommandHint   lipgloss.Style
+	StatusReady   lipgloss.Style
+	StatusWarn    lipgloss.Style
+	StatusError   lipgloss.Style
+	StatusInfo    lipgloss.Style
+	Pill          lipgloss.Style
+	PillAccent    lipgloss.Style
+	Palette       lipgloss.Style
+	PaletteMatch  lipgloss.Style
+	ToolName      lipgloss.Style
+	ToolKey       lipgloss.Style
+	ToolValue     lipgloss.Style
 	UserLabel     lipgloss.Style
 	ToolLabel     lipgloss.Style
 	ErrorLabel    lipgloss.Style
@@ -116,14 +139,15 @@ func (t theme) styles() styles {
 			Foreground(lipgloss.Color(t.Muted)),
 
 		Activity: lipgloss.NewStyle().
-			Bold(true).
-			Foreground(lipgloss.Color(t.Background)).
-			Background(lipgloss.Color(t.Title)).
+			Foreground(lipgloss.Color(t.Muted)).
 			Padding(0, 1),
 
+		Header: lipgloss.NewStyle().
+			Bold(true).
+			Foreground(lipgloss.Color(t.Title)),
+
 		ModelMeta: lipgloss.NewStyle().
-			Foreground(lipgloss.Color(t.Text)).
-			Background(lipgloss.Color(t.SurfaceAlt)).
+			Foreground(lipgloss.Color(t.Muted)).
 			Padding(0, 1),
 
 		Footer: lipgloss.NewStyle().
@@ -141,9 +165,51 @@ func (t theme) styles() styles {
 
 		CommandHint: lipgloss.NewStyle().
 			Foreground(lipgloss.Color(t.Muted)).
+			Padding(0, 1),
+
+		StatusReady: lipgloss.NewStyle().
+			Foreground(lipgloss.Color(t.Assistant)),
+
+		StatusWarn: lipgloss.NewStyle().
+			Foreground(lipgloss.Color(t.Tool)),
+
+		StatusError: lipgloss.NewStyle().
+			Foreground(lipgloss.Color(t.Error)),
+
+		StatusInfo: lipgloss.NewStyle().
+			Foreground(lipgloss.Color(t.User)),
+
+		Pill: lipgloss.NewStyle().
+			Foreground(lipgloss.Color(t.Text)).
 			Background(lipgloss.Color(t.SurfaceAlt)).
 			Padding(0, 1),
 
+		PillAccent: lipgloss.NewStyle().
+			Bold(true).
+			Foreground(lipgloss.Color(t.Title)).
+			Background(lipgloss.Color(t.SurfaceAlt)).
+			Padding(0, 1),
+
+		Palette: lipgloss.NewStyle().
+			Foreground(lipgloss.Color(t.Text)).
+			Border(lipgloss.ThickBorder()).
+			BorderForeground(lipgloss.Color(t.Border)).
+			Padding(0, 2),
+
+		PaletteMatch: lipgloss.NewStyle().
+			Bold(true).
+			Foreground(lipgloss.Color(t.User)),
+
+		ToolName: lipgloss.NewStyle().
+			Bold(true).
+			Foreground(lipgloss.Color(t.Tool)),
+
+		ToolKey: lipgloss.NewStyle().
+			Foreground(lipgloss.Color(t.Muted)),
+
+		ToolValue: lipgloss.NewStyle().
+			Foreground(lipgloss.Color(t.Text)),
+
 		UserLabel:   labelStyle(t.User),
 		ToolLabel:   labelStyle(t.Tool),
 		ErrorLabel:  labelStyle(t.Error),