feat: implement Phase 7 - Signal Operations

Features added:
- Send Unix signals to processes via interactive modal
- Support for common signals (HUP, INT, TERM, KILL, USR1, USR2, QUIT)
- Custom signal input for advanced use cases
- Safety confirmations for dangerous signals (TERM, KILL, QUIT)
- Signal button added to each ProcessCard

Implementation details:
- Created signal API routes:
  - /api/supervisor/processes/[name]/signal - Send signal to process
  - /api/supervisor/groups/[name]/signal - Send signal to group
  - /api/supervisor/processes/signal-all - Send signal to all processes

- Added React Query hooks:
  - useSignalProcess() - Send signal to single process
  - useSignalProcessGroup() - Send signal to process group
  - useSignalAllProcesses() - Send signal to all processes

- Created SignalSender modal component:
  - Grid of common signal buttons with descriptions
  - Custom signal text input (auto-uppercase)
  - Two-step confirmation for dangerous signals
  - Visual warning with AlertTriangle icon
  - Destructive button variant for confirmed dangerous signals
  - Backdrop blur overlay

ProcessCard enhancements:
- Added Zap icon signal button
- Modal opens on signal button click
- Button disabled when process is stopped
- Modal integrates with useSignalProcess hook

Common signals with descriptions:
- HUP (1): Reload configuration
- INT (2): Interrupt - graceful shutdown
- QUIT (3): Quit
- TERM (15): Terminate - graceful shutdown (dangerous)
- KILL (9): Kill - immediate termination (dangerous)
- USR1 (10): User-defined signal 1
- USR2 (12): User-defined signal 2

Safety features:
- Dangerous signals require confirmation
- Warning message explains risks
- Button changes to destructive variant
- Custom signals also checked for danger
- Clear visual feedback during operation

Phase 7 complete (3-4 hours estimated)

lib/hooks/useSupervisor.ts

@@ -558,3 +558,90 @@ export function useRemoveProcessGroup() {
     },
   });
 }
+
+// Signal Operations
+
+async function signalProcess(name: string, signal: string): Promise<{ success: boolean; message: string }> {
+  const response = await fetch(`/api/supervisor/processes/${encodeURIComponent(name)}/signal`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ signal }),
+  });
+  if (!response.ok) {
+    const error = await response.json();
+    throw new Error(error.error || 'Failed to send signal');
+  }
+  return response.json();
+}
+
+async function signalProcessGroup(name: string, signal: string): Promise<{ success: boolean; message: string; results: any[] }> {
+  const response = await fetch(`/api/supervisor/groups/${encodeURIComponent(name)}/signal`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ signal }),
+  });
+  if (!response.ok) {
+    const error = await response.json();
+    throw new Error(error.error || 'Failed to send signal to group');
+  }
+  return response.json();
+}
+
+async function signalAllProcesses(signal: string): Promise<{ success: boolean; message: string; results: any[] }> {
+  const response = await fetch('/api/supervisor/processes/signal-all', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ signal }),
+  });
+  if (!response.ok) {
+    const error = await response.json();
+    throw new Error(error.error || 'Failed to send signal to all processes');
+  }
+  return response.json();
+}
+
+export function useSignalProcess() {
+  const queryClient = useQueryClient();
+
+  return useMutation({
+    mutationFn: ({ name, signal }: { name: string; signal: string }) => signalProcess(name, signal),
+    onSuccess: (data, variables) => {
+      toast.success(data.message);
+      queryClient.invalidateQueries({ queryKey: supervisorKeys.process(variables.name) });
+      queryClient.invalidateQueries({ queryKey: supervisorKeys.processes() });
+    },
+    onError: (error: Error) => {
+      toast.error(`Failed to send signal: ${error.message}`);
+    },
+  });
+}
+
+export function useSignalProcessGroup() {
+  const queryClient = useQueryClient();
+
+  return useMutation({
+    mutationFn: ({ name, signal }: { name: string; signal: string }) => signalProcessGroup(name, signal),
+    onSuccess: (data) => {
+      toast.success(data.message);
+      queryClient.invalidateQueries({ queryKey: supervisorKeys.processes() });
+    },
+    onError: (error: Error) => {
+      toast.error(`Failed to send signal to group: ${error.message}`);
+    },
+  });
+}
+
+export function useSignalAllProcesses() {
+  const queryClient = useQueryClient();
+
+  return useMutation({
+    mutationFn: (signal: string) => signalAllProcesses(signal),
+    onSuccess: (data) => {
+      toast.success(data.message);
+      queryClient.invalidateQueries({ queryKey: supervisorKeys.processes() });
+    },
+    onError: (error: Error) => {
+      toast.error(`Failed to send signal to all processes: ${error.message}`);
+    },
+  });
+}