feat: add session resume capability with automatic continuation

Add ability to resume inactive sessions and automatically continue with new
session ID. Includes message merging for preserving conversation history,
UI improvements to allow sending during inactive state, and database schema
migration. close #87

web/src/components/SessionChat.tsx

@@ -40,7 +40,7 @@ export function SessionChat(props: {
 }) {
     const { haptic } = usePlatform()
     const navigate = useNavigate()
-    const controlsDisabled = !props.session.active
+    const sessionInactive = !props.session.active
     const normalizedCacheRef = useRef<Map<string, { source: DecryptedMessage; normalized: NormalizedMessage | null }>>(new Map())
     const blocksByIdRef = useRef<Map<string, ChatBlock>>(new Map())
     const [forceScrollToken, setForceScrollToken] = useState(0)
@@ -250,7 +250,8 @@ export function SessionChat(props: {
         isSending: props.isSending,
         onSendMessage: handleSend,
         onAbort: handleAbort,
-        attachmentAdapter
+        attachmentAdapter,
+        allowSendWhenInactive: true
     })
 
     return (
@@ -263,10 +264,10 @@ export function SessionChat(props: {
                 onSessionDeleted={props.onBack}
             />
 
-            {controlsDisabled ? (
+            {sessionInactive ? (
                 <div className="px-3 pt-3">
                     <div className="mx-auto w-full max-w-content rounded-md bg-[var(--app-subtle-bg)] p-3 text-sm text-[var(--app-hint)]">
-                        Session is inactive. Controls are disabled.
+                        Session is inactive. Sending will resume it automatically.
                     </div>
                 </div>
             ) : null}
@@ -278,7 +279,7 @@ export function SessionChat(props: {
                         api={props.api}
                         sessionId={props.session.id}
                         metadata={props.session.metadata}
-                        disabled={controlsDisabled}
+                        disabled={sessionInactive}
                         onRefresh={props.onRefresh}
                         onRetryMessage={props.onRetryMessage}
                         onFlushPending={props.onFlushPending}
@@ -296,11 +297,12 @@ export function SessionChat(props: {
                     />
 
                     <HappyComposer
-                        disabled={props.isSending || controlsDisabled}
+                        disabled={props.isSending}
                         permissionMode={props.session.permissionMode}
                         modelMode={props.session.modelMode}
                         agentFlavor={agentFlavor}
                         active={props.session.active}
+                        allowSendWhenInactive
                         thinking={props.session.thinking}
                         agentState={props.session.agentState}
                         contextSize={reduced.latestUsage?.contextSize}