perf: reduce streaming render cost and refresh activity page on demand

- Segment streaming Markdown so completed blocks are reused and only the growing tail is re-parsed; fall back to whole-document parsing for document-wide constructs
- Memoize conversation sidebar rows and cache per-conversation derived activity, ordering and pending approvals
- Activity page loads on entry, refreshes every 15 s while visible, and has a single header refresh button
- Include pending settings, model download source, device sharing and knowledge gateway changes in the working tree
- Add Electron streaming input benchmark and regression tests

src/shared/application-settings-contracts.ts

@@ -39,7 +39,7 @@ export const supervisionTimeoutSecondsSchema = z.number().int().min(30).max(600)
 
 export const builtInApplicationIds = ['magic-notes', 'knowledge', 'heartbeat', 'local-inference', 'device-sharing'] as const
 export type BuiltInApplicationId = typeof builtInApplicationIds[number]
-export const editableApplicationIds = ['magic-notes', 'heartbeat', 'local-inference'] as const
+export const editableApplicationIds = ['magic-notes', 'heartbeat', 'local-inference', 'device-sharing'] as const
 export type EditableApplicationId = typeof editableApplicationIds[number]
 export const defaultApplicationNavigation = {
   order: ['knowledge', 'heartbeat', 'magic-notes', 'local-inference', 'device-sharing'] as BuiltInApplicationId[],
@@ -67,6 +67,7 @@ const applicationPreferencesSchema = z
     remoteProjectsEnabled: z.boolean(),
     applicationNavigation: applicationNavigationSchema.default(defaultApplicationNavigation),
     localInferenceEnabled: z.boolean().default(true),
+    deviceSharingEnabled: z.boolean().default(false),
     magicNotesEnabled: z.boolean().default(true),
     heartbeatEnabled: z.boolean().optional(),
     heartbeatReportTimeoutSeconds: supervisionTimeoutSecondsSchema.optional(),
@@ -93,6 +94,7 @@ export const applicationSettingsUpdateSchema = applicationPreferencesSchema
     desktopNotificationsEnabled: z.boolean().optional(),
     transparentFrostedEffectEnabled: z.boolean().optional(),
     localInferenceEnabled: z.boolean().optional(),
+    deviceSharingEnabled: z.boolean().optional(),
     magicNotesEnabled: z.boolean().optional(),
     heartbeatEnabled: z.boolean().optional(),
     magicNotesShowIncompleteTodoCount: z.boolean().optional(),

src/shared/goodbuddy-config-contracts.test.ts

@@ -159,6 +159,7 @@ describe('GoodBuddy configuration contracts', () => {
         localToolEnvironment: defaultLocalToolEnvironmentSettings,
         applicationNavigation: defaultApplicationNavigation,
         localInferenceEnabled: true,
+        deviceSharingEnabled: false,
         conversationHtmlRenderingEnabled: true,
         remoteProjectsEnabled: false,
         magicNotesEnabled: true,