Fix L17/L18/L56: regex validation, terminal visibility, SponsorBlock warning

L17: TemplateManager urlPattern field now validates the regex on each keystroke;
     an error message appears below the input and Save is blocked until fixed.
L18: Terminal nav item is hidden when customCommandEnabled is false -- no more
     dead-end gated view. Sidebar gains showTerminal prop; App.tsx reads the
     setting and passes it through.
L56: SponsorBlock Categories Field shows a 'warning' validationMessage when the
     toggle is ON but no categories are selected, making the silent no-op visible.

typecheck + 242 tests + eslint + prettier green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CODE-AUDIT.md

@@ -410,9 +410,9 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
   already that union).
 - [ ] **L16 — `relTime`/`formatWhen` verbosity.** `relTime` returns unbounded "N d ago"; History's
   `formatWhen` always appends the year, even for the current year.
-- [ ] **L17 — TemplateManager regex not validated.** An invalid `urlPattern` is accepted with no
+- [x] **L17 — TemplateManager regex not validated.** An invalid `urlPattern` is accepted with no
   feedback and silently never matches (main's `matchesUrl` swallows the error).
-- [ ] **L18 — Terminal nav item shown when custom commands are off** — leads to a gated dead-end
+- [x] **L18 — Terminal nav item shown when custom commands are off** — leads to a gated dead-end
   view; consider hiding/disabling it.
 - [ ] **L19 — `App.tsx` focus hack** — `setTimeout(() => …focus(), 60)` after a tab switch.
 - [ ] **L20 — `youtubePlayerClient` is free text** with no allowlist/validation; a typo passes
@@ -486,7 +486,7 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
 - [x] **L54 — DownloadBar preview `<img>` has no `onError` fallback** (inconsistent with `MediaThumb`'s retry).
 - [x] **L55 — QueueItem meta line can show size twice** — a probed-format `quality` label already
   contains the size, then `sizeLabel` is appended again.
-- [ ] **L56 — SponsorBlock ON with zero categories silently no-ops** (`buildArgs` guards on
+- [x] **L56 — SponsorBlock ON with zero categories silently no-ops** (`buildArgs` guards on
   `length > 0`) with no UI warning.
 - [x] **L57 — Scheduling a past datetime silently downloads now** (`buildItem` future-only guard),
   no feedback that the schedule was ignored.

src/renderer/src/App.tsx

@@ -38,6 +38,7 @@ function App(): React.JSX.Element {
   const theme = useSettings((s) => s.theme)
   const accentColor = useSettings((s) => s.accentColor)
   const updateSettings = useSettings((s) => s.update)
+  const showTerminal = useSettings((s) => s.customCommandEnabled)
   const isDark = useResolvedDark()
   const [tab, setTab] = useState<TabValue>('downloads')
   const [paletteOpen, setPaletteOpen] = useState(false)
@@ -171,6 +172,7 @@ function App(): React.JSX.Element {
               version={version}
               collapsed={collapsed}
               onToggleCollapsed={toggleCollapsed}
+              showTerminal={showTerminal}
             />
 
             <main className={styles.content}>

src/renderer/src/components/DownloadOptionsForm.tsx

@@ -196,7 +196,15 @@ export function DownloadOptionsForm({ value, onChange }: Props): React.JSX.Eleme
               onChange={(v) => setOpt('sponsorBlockMode', v as SponsorBlockMode)}
             />
           </Field>
-          <Field label="Categories">
+          <Field
+            label="Categories"
+            validationState={value.sponsorBlockCategories.length === 0 ? 'warning' : 'none'}
+            validationMessage={
+              value.sponsorBlockCategories.length === 0
+                ? 'No categories selected — no segments will be skipped.'
+                : undefined
+            }
+          >
             <div className={styles.categoryGrid}>
               {SPONSORBLOCK_CATEGORIES.map((cat) => (
                 <Checkbox

src/renderer/src/components/Sidebar.tsx

@@ -152,6 +152,8 @@ interface SidebarProps {
   version: string
   collapsed: boolean
   onToggleCollapsed: () => void
+  /** Show the Terminal nav item (only when custom commands are enabled) */
+  showTerminal: boolean
 }
 
 export function Sidebar({
@@ -162,7 +164,8 @@ export function Sidebar({
   onSetTheme,
   version,
   collapsed,
-  onToggleCollapsed
+  onToggleCollapsed,
+  showTerminal
 }: SidebarProps): React.JSX.Element {
   const styles = useStyles()
   const focus = useFocusStyles()
@@ -214,7 +217,7 @@ export function Sidebar({
       </div>
 
       <div className={styles.nav}>
-        {NAV.map((n) => {
+        {NAV.filter((n) => n.value !== 'terminal' || showTerminal).map((n) => {
           const active = tab === n.value
           const btn = (
             <button

src/renderer/src/components/TemplateManager.tsx

@@ -1,6 +1,7 @@
 import { useState } from 'react'
 import {
   Button,
+  Field,
   Input,
   Textarea,
   Caption1,
@@ -103,10 +104,21 @@ export function TemplateManager(): React.JSX.Element {
     setDraft({ id: t.id, name: t.name, args: t.args, urlPattern: t.urlPattern ?? '' })
   }
 
+  function patternError(): string | undefined {
+    const p = draft?.urlPattern.trim()
+    if (!p) return undefined
+    try {
+      new RegExp(p)
+      return undefined
+    } catch {
+      return 'Invalid regex — this pattern will never match.'
+    }
+  }
+
   function commit(): void {
     if (!draft) return
     const name = draft.name.trim()
-    if (!name) return
+    if (!name || patternError()) return
     const urlPattern = draft.urlPattern.trim()
     save({
       id: draft.id ?? newId('tpl'),
@@ -165,11 +177,16 @@ export function TemplateManager(): React.JSX.Element {
             resize="vertical"
             onChange={(_, d) => setDraft({ ...draft, args: d.value })}
           />
+          <Field
+            validationState={patternError() ? 'error' : 'none'}
+            validationMessage={patternError()}
+          >
             <Input
               value={draft.urlPattern}
               placeholder="Auto-apply to URLs matching (regex, optional) — e.g. soundcloud\.com"
               onChange={(_, d) => setDraft({ ...draft, urlPattern: d.value })}
             />
+          </Field>
           <div className={styles.formActions}>
             <Button appearance="subtle" icon={<DismissRegular />} onClick={() => setDraft(null)}>
               Cancel