feat(tui): add clipboard copy keybinding via OSC 52 (#433)

Press 'c' on Recent Observations, Search Results, Observation Detail,
and Session Detail screens to copy the selected observation content to
the system clipboard using the OSC 52 terminal escape sequence.

Visual feedback ("✓ Copied!") is shown for 2 seconds after each copy.
No new dependencies added — uses the existing go-osc52/v2 transitive dep.

Closes #141

Author: Alan-TheGentleman

internal/tui/clipboard.go

@@ -0,0 +1,46 @@
+package tui
+
+import (
+	"encoding/base64"
+	"fmt"
+	"time"
+
+	tea "github.com/charmbracelet/bubbletea"
+)
+
+// ─── Messages ─────────────────────────────────────────────────────────────────
+
+// clipboardCopiedMsg is sent after building the OSC 52 sequence to copy.
+// The sequence field carries the raw escape sequence so Update can emit it.
+type clipboardCopiedMsg struct {
+	sequence string
+}
+
+// clipboardClearMsg is sent after the 2-second feedback timer expires.
+type clipboardClearMsg struct{}
+
+// ─── OSC 52 helpers ──────────────────────────────────────────────────────────
+
+// osc52Sequence returns the OSC 52 terminal escape sequence that asks the
+// terminal to write content to the system clipboard.
+//
+// Format: ESC ] 52 ; c ; <base64> BEL
+func osc52Sequence(content string) string {
+	b64 := base64.StdEncoding.EncodeToString([]byte(content))
+	return fmt.Sprintf("\x1b]52;c;%s\x07", b64)
+}
+
+// copyToClipboard returns a Cmd that builds the OSC 52 sequence for content
+// and wraps it in a clipboardCopiedMsg.
+func copyToClipboard(content string) tea.Cmd {
+	return func() tea.Msg {
+		return clipboardCopiedMsg{sequence: osc52Sequence(content)}
+	}
+}
+
+// clearFeedbackAfter returns a Cmd that sends clipboardClearMsg after d.
+func clearFeedbackAfter(d time.Duration) tea.Cmd {
+	return tea.Tick(d, func(_ time.Time) tea.Msg {
+		return clipboardClearMsg{}
+	})
+}