docs: comprehensive architecture documentation with PlantUML diagrams

- docs/ARCHITECTURE.md: full system overview, technology stack, directory
  structure, component architecture, data flow, auth, polling/caching,
  download matching pipeline, API reference, frontend architecture,
  configuration, deployment guide
- docs/diagrams/component.puml: system component diagram
- docs/diagrams/seq-auth.puml: authentication sequence diagram
- docs/diagrams/seq-dashboard.puml: dashboard request sequence diagram
- docs/diagrams/seq-polling.puml: background polling cycle sequence
- docs/diagrams/class-server.puml: server-side class/module diagram
- docs/diagrams/class-data.puml: data model / entity diagram
- docs/diagrams/state-ui.puml: frontend UI state diagram
- docs/diagrams/state-poller.puml: poller state diagram
- docs/diagrams/activity-matching.puml: download matching activity diagram

docs/diagrams/state-ui.puml

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+@startuml state-ui
+!theme plain
+title sofarr — Frontend UI State Diagram
+
+[*] --> SplashScreen : Page load
+
+state SplashScreen {
+  state "Showing splash\n(min 1.2s)" as showing
+}
+
+SplashScreen --> CheckAuth : checkAuthentication()
+
+state CheckAuth <<choice>>
+CheckAuth --> LoginForm : No session cookie
+CheckAuth --> Dashboard : Valid session
+
+state LoginForm {
+  state "Idle" as lf_idle
+  state "Submitting" as lf_submit
+  state "Error" as lf_error
+
+  lf_idle --> lf_submit : Submit form
+  lf_submit --> lf_error : Auth failed
+  lf_error --> lf_submit : Re-submit
+  lf_submit --> FadeOutLogin : Auth success
+}
+
+state FadeOutLogin {
+  state "CSS transition\n(opacity → 0)" as fade
+}
+
+FadeOutLogin --> SplashScreen2 : Show splash\nwhile loading
+
+state SplashScreen2 as "Splash (loading data)" {
+  state "fetchUserDownloads()" as fetching
+}
+
+SplashScreen2 --> Dashboard : Data loaded\ndismissSplash()
+
+state Dashboard {
+  state "Rendering Cards" as rendering
+  state "Auto Refreshing" as refreshing
+  state "Status Panel Open" as status_open
+  state "Status Panel Closed" as status_closed
+
+  [*] --> rendering
+  rendering --> refreshing : startAutoRefresh()
+  refreshing --> rendering : fetchUserDownloads()\n→ renderDownloads()
+  rendering --> rendering : Theme change
+
+  status_closed --> status_open : Click "Status" btn\n(admin only)
+  status_open --> status_closed : Click close (×)
+  status_open --> status_open : Auto-refresh\nrenderStatusPanel()
+
+  [*] --> status_closed
+
+  state "Refresh Rate" as rr {
+    state "1s" as r1
+    state "5s (default)" as r5
+    state "10s" as r10
+    state "Off" as roff
+    r5 --> r1 : User selects
+    r5 --> r10
+    r5 --> roff
+    r1 --> r5
+    r1 --> r10
+    r1 --> roff
+    r10 --> r1
+    r10 --> r5
+    r10 --> roff
+    roff --> r1
+    roff --> r5
+    roff --> r10
+  }
+}
+
+Dashboard --> LoginForm : Logout\n(clear cookie,\nstopAutoRefresh)
+
+@enduml