feat: Activity feed — cron deliveries with output (#420)

* feat: add Activity feed showing cron jobs that produced output

- New ActivityItem data model for unified notification feed
- SessionInfo extended with preview/started_at fields
- GET /api/sessions?source=cron endpoint added with source param
- ActivityViewModel fetches cron run sessions, filters for output
- ActivityScreen shows chronological feed with status icons
- Wired into navigation (AUTOMATE drawer section)
- Only shows runs with message_count > 0 (signal-driven)

Closes #402

* fix: add @Composable to statusColor helper (MaterialTheme access)

* fix: Double type for started_at (backend returns float timestamps)

* fix: use GET /api/cron/jobs for Activity feed (no-agent jobs don't create sessions)

Root cause: no-agent cron jobs (jellyfin, battery monitor, etc.) run
scripts directly and NEVER create DB sessions. The previous approach
using GET /api/sessions?source=cron only showed 3 old agent-driven
sessions.

Fix: use GET /api/cron/jobs which tracks last_run_at + last_status for
ALL job types. Filter last_status == "ok" (job ran and delivered).

* fix: match codebase patterns for ViewModel, Scaffold, and NavIcon

- safeApiCall returns NetworkResult, use when(is) not onSuccess/onFailure
- HermesScaffold navigationIcon expects NavIcon.Menu/Back sealed class
- Screen composable param must be onOpenDrawer (capital O)

* fix: correct ActivityScreen API mismatches

- NavIcon.Menu param is onOpen, not onClick
- EmptyState param is subtitle, not description
- Add missing viewModel import

* fix: padding, clickable cards, and navigation on Activity screen

- Fix double padding by using listContentPadding + listItemSpacing
  matching the codebase pattern
- Add clickable cards that navigate to CronJobsScreen
- Remove unused scroll and spacer workarounds

* refactor: replace separate Activity feed with delivered-only filter in Cron tab

- Remove ActivityScreen, ActivityViewModel, ActivityItem
- Remove ActivityScreen from navigation keys and registry
- Add showDeliveredOnly toggle to CronJobsUiState
- Add TaskAlt icon button in cron top bar toggles filter
- Jobs with lastRunStatus != "ok" hidden when filter is on

* feat: clickable cron job cards show run-details dialog

- Clicking a job card opens an AlertDialog with last run info
- Shows status, last run time, schedule, error (if any), script
- RunDetailRow helper composable for label/value pairs

* fix: add missing CronJob import in CronJobsScreen

* fix: use snake_case field names for CronJob model

- lastRunAt -> last_run_at, lastError -> last_error
- These are data class constructor params, not computed properties

* refactor: remove delivered-only filter toggle, keep card-click dialog

- Remove TaskAlt filter icon button from Cron tab top bar
- Remove showDeliveredOnly from state and toggleDeliveredFilter() method
- Simplify cron list back to direct state.jobs with no filter layer

* chore: remove dead activity_empty strings

---------

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/data/model/CronJob.kt

@@ -15,6 +15,9 @@ data class CronJob(
     val schedule_display: String? = null,
     val last_status: String? = null,
     val next_run_at: String? = null,
+    val last_run_at: String? = null,
+    val last_error: String? = null,
+    val last_delivery_error: String? = null,
     // Full editor fields — all optional with defaults for backward compat
     val enabled: Boolean? = null,
     val prompt: String? = null,

app/src/main/java/com/m57/hermescontrol/data/model/SessionListResponse.kt

@@ -13,4 +13,6 @@ data class SessionInfo(
     val created_at: String?,
     val message_count: Int?,
     val status: String?,
+    val preview: String? = null,
+    val started_at: Double? = null,
 )